The people who do this will be a vanishingly small minority. It's not as easy to set up one's own modem as it is their own router, IME. And even then, going with your own router is rare.
I mean, I suppose it's got the additional step of calling Comcast and giving them the MAC of your modem, but IIRC that's all I had to do after buying one on their approved list. Been at least 7-8 years since I had them, though.
You can plug-and-play with a consumer "router", but even then you need to know the difference between WAN and LAN sides. So the extra effort seems minimal.
Most people don't know how to set up either one. I know when the fiber techs came to my house to set me up they were greatly impressed at my (fairly basic; I don't do this for a living) networking knowledge.
I have been using my setup for 9 years and never paid a dime in rent of equipment.
the setup is plug-n-play / one youtube video away