Thank endless lobbying that make legally mandated monopolies a thing in this arena. They did it with phone companies too.
It's not "just Comcast" by happenstance. It's "just Comcast" by legal regulation.
Funny that you complain about Comcast, when I was with Comcast I actually had native IPv6. With my current provider I only have IPv6 through NAT46.
Maybe Starlink is another option. It has some drawbacks like reduced performance during heavy rain, but I've seen some positive reviews as well.
Most people don't realize there are two IPv6 internets right now, the Cogent side and the Hurricane Electric side. Both are equally sized and refuse to connect to each other, so you need to know that and either buy transit from both or buy transit from a network that buys from both. At least one major provider I know of is still running v6 over tunnels. In many places your v6 traffic is taking suboptimal routes, whereas an enterprise network may have v4 connectivity at each datacenter, v6 all gets sent to that one box under Dave's desk.
But we continue to measure v6 adoption at places like Google and Cloudflare where dedicated teams make sure packets arrive and pat ourselves on the back.
Cogent engages in peering spats on IPv4 too; this dynamic is not new with or unique to IPv6, or limited to Cogent/HE. The lesson here is to not go singlehomed under Cogent, not to reject IPv6.
Currently v6 is like connecting to the late 90s internet. It dosen't work as well as people think.