There are, undoubtedly, many reasons for justifying not having to use their equipment.
Besides rental cost, having extraneous hardware which is unused functionality at least raises the probability that there could be something which goes wrong due to added complexity.
The most compelling reason being if I insist on using bridge mode(on such a gateway), and then, after some unforseen firmware upgrade, that setting is reset, then my entire network becomes unreachable. Or at least as unreachable as it once was before the update.
Having a simple bridge device like a pure modem or plain old ont, there can be no functionality to reset which would potentially alter the state of the netowrk. It either passes as a bridge or it doesnt.
A lot of the friction though, at its core is the result of either, drumroll, having full access to the device providing layer 3 NAT or not.
As ISPs want to smush together their on premises equipment they, due to the nature of the stack need to take control of the NAT to do so. At that point users who would like to open ports or do anything more than request a connection from the insdide are, out of luck, and it shoulnd't be accepted, as ISPs dont NEED gateways to make thier network work. Illustrated by the many smaller ones who do just fine without.