My uneducated guess is money.

Manufacturers likely have factories (either theirs or a contracted company’s) where they can mass produce the power bricks for a low cost. Upgrading to a USB power supply doesn’t offer significant benefits compared to the power brick of similar wattage, and the up-front cost of setting up a new supplier is financially unjustified. The old technology works just as well, so why change?

High power USB is still a relatively new technology. I’m sure it will proliferate, but the consumer market has a fuckton of inertia.