It’s because this kind can be shipped deflated and easily filled by the shipper using a small desktop machine that inflates it from the edge. That way they don’t have to pay to ship and store giant rolls of mostly air. It quickly pays for the cost of the machine.

That’s how it is in the US. Sometimes you may have to disconnect and connect things underneath with limited clearance, and usually have to disconnect the drain tube on the sink side. It may be screwed into the countertop. There are leveling feet to adjust. It also may or may not be hard wired. It just takes a few minutes in theory, but tight clearances can make it a pain.