I always see laptops come with larger bricks that have separate mains cable which plugs into the brick. Even if the adapter has some low power rating.
Example for comparison:
On the right is a standard laptop adapter.
Same price, same connector, same protocol, same power rating, far different size.
Why is that so?
Actually, the GaN adapter on the left also advertises itself as being meant for laptops, but by default, almost all laptops will come with something like the one on the right.
Or is it simply cheaper to manufacture while being sold for the same price?
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.