It's not done because the market won't support it - there is far less profit and demand for training (which actually takes longer) than for performance. Also, there is too much specialized knowledge involved to pass on easily and quickly enough to make it cost effective for a small or even medium project, and organized teaching is not a core competency of most tradespeople.
On a tangential note, your needs as a homeowner are very different from the needs of a contractor trying to make a living. The construction industry is weird; construction customer's expectations are completely unacceptable in almost any other industry. Clients expect a retain an expert with extensive training and specialized tools to provide a service, while also expecting to negotiate rates, receive training, perform work themselves, take control over the supply chain, act as project management, and change the scope of the project after it starts - without taking any responsibility for the outcome.
Imagine walking into a restaurant with a raw steak, asking them to teach you to prepare it in their kitchen with a knife you brought, and then allow you to seat and serve yourself - all for 1/3 the menu price because 'I bought the food' and 'the staff didn't really do anything to it'.
Discussion / rambling:
While I see the potential benefit of a "DIY advisor" to both parties, it would be very difficult to implement. Few homeowners want to actually learn trade skills, and a full-time contractor can't survive on the few 'advisor' jobs they might get. Their most profitable path is to getting the projects they specialize in, not branching out into learning how to be a teacher and traveling to multiple job sites without being able to spend the day there earning revenue.
You're asking someone who has spent a lot of time learning a skill, focusing on learning how to evaluate and perform a wide range of tasks needed to complete a project, to instead undercut themselves by evaluating the project, passing on the knowledge needed for a project, and help with managing the project - while working with someone who doesn't know what they're doing, and without getting paid for doing the project.
They can't do this profitably without charging a premium for their management skills and domain experience (which is what you're really paying a contractor for anyways) - but since residential-scale projects are nearly always priced as 'time and materials', you buying the materials and providing 'all' the labor leaves nothing to hang a price tag on. And the project will take longer to do because the labor involved (you) is inexperienced. So an 'advisor' job is going to be the same or more work to get the project kicked off, with none of the pay for doing the work.
Now, viewing such an arrangement as "education" sounds reasonable - but I doubt most homeowners would place a reasonable value on the education. They don't want to put in the time and money for a trade school education or get hired on as an entry-level employee, they want to hand over an hour's fee to a contractor in exchange for the knowledge to guide them through several days of widely varying tasks. Nobody teaches or learns that quickly.
I realize the numbers below are all very squishy and can be adjusted to show whatever you want, but writing this helped me think through the pros and cons so I decided to leave it in.
Imagine a bid for turning a fair-sized DIY task from a performance project into an advising project - for example, adding a small bathroom into an unfinished basement. The contractor travels to the site, talks through the project, creates a task list of the various stages, specifies the exact type and quantity of materials needed, explains the relevant sections of the plumbing and electrical codes in detail. Then at each stage of the project, they travel back to demonstrate the technique and check the work done (and probably explain how to correct it), until the project is finished. The only thing the contractor hasn't done is purchase and transport the materials, or stand there and do the "work" with their own hands.
Now if the homeowner is a fast learner and working on this full time, maybe they knock the actual work out as fast as the contractor would've, say, 10 days. At this point the contractor has been by 4-5 times on his rounds and managed all of the project's overhead tasks. In total, call it only 3 days of planning, travelling, teaching, and overseeing a project that would've also taken 10 days for them.
Now let's run the numbers. Call it a $10K bid as a regular project for easy math, $5K each for labor and materials (including acquisition, markup, and transportation), so an average $500/day for the contractor's expertise and labor after material expenses.
Instead though, this is now an advisory job because the homeowner wants to pay themselves instead. From that same $10K, the homeowner is going to spend $3500 on materials (and ignore their personal cost of time and vehicle mileage to the store), and expect to pocket $1500 there. On the labor side, the homeowner expects to pay nothing because they are doing the 'work', but maybe this homeowner is reasonable and allocates themselves only 60% of what the contractor would have charged, or $300/day, for those same 10 days, or $3000. This leaves $2000 from the labor budget. So for the contractor's 3 days involvement, as much as $667/day to 'boss the job'. So the homeowner ends up paying only $3500 + $2000 = $5500 for the project, and the contractor made an extra $500 over the three days he was involved. Hooray, everyone wins!
Wrong. The homeowner also spent those same 3 days learning the job and working with the contractor besides the 10 days of actual work, and probably had to buy $500 in tools they didn't already own for plumbing/electrical/tile/cabinetry (again ignoring their own cost of wear and tear on personal equipment). 13 days @ $300 = $3900, plus tools = $4400 out of the $5K labor budget, leaving $600 for the contractor, who now only made $200/day = $600 for 3 days, instead of $1500, a $900 loss. Even taking tools out of the material budget instead is $1100 over 3 days = $367/day, or $400 less than the original fixed fee rate.
And that's before anything goes wrong with the project and the homeowner changes their original scope, or cancels halfway through, or takes 20 days instead of 10 and pushes the remaining billable consults out past the day the contractor needs to make payroll...