What is the folly of this method?
If the "only" thing that were to happen was that one of these DIY plates failed with you on the deck (and nobody else), you aren't likely to sue yourself, but the medical bills could easily more than overwhelm the difference in price between the DIY solution and the approved solution.
Is it dangerous?
You do not have the structural engineering skills/knowledge to design this junction properly and to code specifications (if you did, you wouldn't be asking a bunch of strangers on the internet), therefore, it's quite likely that it will be underdesigned and that could be quite dangerous.
What makes the commercial product better than just purchasing steel plates and bolts?
The commercial product has behind it a team of lawyers who will understand and interpret the legal aspects of the building code and a team of structural engineers who will interpret the mechanical and structural aspects of the code. They will work together to design a product that will meet or exceed the minimum requirements. They then have a team of manufacturing engineers who will ensure that the product is built according to specification and a team of quality engineers to ensure that the output of the manufacturing process actually meets the design specification.
You are purchasing some steel plates and bolts, then asking some strangers if that'll do.
Additionally, you'll find that while sandwiching a piece of ply between 2x material is common practice for building indoor headers above doors and windows since it provides an even surface on both sides for applying finishing material, it's not particularly common in outdoor deck construction. At a minimum, you'll be exposing the end/edge grain of the plywood to moisture and trapping it there. It will sit there until it evaporates giving it maximum time to penetrate into the most vulnerable part of the wood, increasing the rate at which the plywood will fail. As the plywood fails, your joists will become loose and begin to fail. As a matter of fact, you probably won't find commercially manufactured post caps (designed for holding the beams on top of the posts) that will accept your dimensional/plywood sandwich - it will be too thick.