The normal Comcast plans do let you do that, this is some weird special plan. You can hook up a standard DOCSIS modem and then DHCP lease an IP.

There are a lot of technically-proficient people who refuse to do this anymore. I guess it is a non-zero amount of work, but it really, really isn't that much. The direction we're headed in is pretty obvious: some day, sooner rather than later, the era of customers being able to use their own cable modems for DOCSIS internet will slowly end, as the people who bother become more and more in the minority.