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?
People carry around laptops in bags. People carry around their phones in their pockets. This is another reason why mobile chargers are often smaller. They need to be, because it’s more likely that you’ll be carrying a phone charger in your pocket than in a laptop bag.
Also, making a powerful adapter smaller costs more money. It’s a way to lower the overall price of a laptop.