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?
GaN is newer technology and more expensive to manufacture. It can power laptops like anything else just in a much smaller package. I have a 65W GaN charger that charges my work laptop while on the go just as quickly as the official brick I leave in the office. You just need to pair it with a USB-PD rated cable that will handle the wattage your laptop requires. In my case, I ended up getting a 240W USB-C JSAUX cable. It’s stiff but does the job.
This is the main factor.
The second factor is manufacturer/model specific. Cleaning up the wave after conversion to DC is handled by the power adapter down to +/-5% tolerance for PD, for example. Manufacturers with lower tolerance designs will sometimes move the hardware for any additional refinement of the wave to the power adapter in order to reduce the size of the device.