Actually, they do.
Look at the packages for "Key Numbers"
Every lock package will have a prominent 3-5 digit "Key Number" somewhere on the outside of the package, typically the back. All packages with the same key number use the same key.
So if you have 3 knobs and 2 deadbolts you all want keyed alike, buy 3 knob+deadbolt sets of the same number.
Generally there are 3-8 lock packages in a carton (all with the same style/color/SKU). Every package in the carton will have the same key number. Search the packages with the same SKU until you find your matches. If not enough match, see if they have an unopened carton on the racks, or hit another big-box store. (This is the one time I'll recommend big-box; they buy whole cartons).
This only works within the same SKU (barcode number). Do not turn the store upside down looking for the same key number on a different SKU, you won't find it. So if you need 3 knobs + 2 deadbolts, you must buy 3 knob+deadbolt sets, not 2 and a loose knob.
Speaking of security... Buy your lock-sets far away, at places you don't normally shop. Once we demoted someone in a club and changed the knob/lock to his office. He drilled out ours and fit his own knob-lock. I noted the model, went to the local hardware store and bought one of each key number. Voilà. He could never figure out how I did it!
Some brands of locks are self-rekeyable. But they're cheap.
Certain low-midline brands of lock allow you to "re-key" the lock to any arbitrary key. You unlock it, insert a special tool, and then insert the new desired key. These are compatible with certain keyways. I'm not a fan, honestly; the mechanism seems flimsy given the price point.
Your friendly neighborhood locksmith can do anything
They can make anything you want. But the price will be nosebleed compared to the other options. They can cheerfully build any lock-set (even a padlock!) to match your key.
First, there are 3 kinds of locksmiths:
- Scammers who flood Google. They collect your contact info and sell it to shady "outbound only" locksmiths who come to you, overcharge you, and/or case your house for a later break-in.
- Outbound-only. They have a truck, not a shop, and their business is the urgent call and collecting that service charge for coming to you.
- Shop-based locksmiths (it need not be neat). They are happy for you to come to them, and can make anything since they have a well-equipped shop. They cheerfully handle routine requirements at sensible cost (for locksmiths).
So when calling around, if they pressure you to come to you, hang up the phone: that's a fly-by-night with a truck. Going to their well-worn location assures they're the real deal. You can also find them in 10-year-old phone books.