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.
What are you talking about? Modems are incredibly simple to set up. You buy it, log into your account on another network or call the ISP, enter your modem's mac address...and that's it. You have to type in the mac or read it off over the phone. There's nothing on device to set up, it's much easier than a router.
You also get better rates if you use their equipment.
> It's not as easy to set up one's own modem as it is their own router, IME.
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.