Earlier quoted context omitted.
When I've had a cable modem, all I've had to do is connect my router to the modem via Ethernet. When I've had modemless service via Ethernet jack, all I've had to do is connect my router to the wall Ethernet jack. So what are you talking about? Purchasing your own cable modem? Or something else that I've never had to do?
If you have DSL, the modem and the router are the same box, so if you want your own, you have to throw out the one the ISP gave you, and configure yours by yourself. In practice, I bet you could just call up the ISP if you're having trouble, but I agree that an ISP has an incentive to give you a good unit to start with.
The usual recommended approach I've seen is to keep the ISP-provided modem/router, plug your router into it, and make certain configuration changes on one or both of the devices (though, for the same ISP, I've seen conflicting information on the correct configuration changes, with clearly dated assumptions, but still possibly correct as far as it goes information, in the official information from the ISP.)
But my basic point is that for most home users, they aren't conceptualizing it as spending separately on "internet service package" and "network hardware", the network hardware is part of the internet service package.