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?
Pretty much this.
If you’ve ever tried using a phone charger to continuously power something at or near its rated maximum output, you’ll find that after a while the power becomes unstable and the charger often quite warm.
e.g. Continuously powering a Raspberry Pi or USB powered computer monitors.