There's a good reason: The demand for it is infinitesimally tiny, and the hassle of doing so (and having to deal with monitoring whether or not you end up being a spam source) is an extra cost.

An argument that it's not "full" internet access will get you nowhere - very few consumer internet subscriptions are "full" internet access in the sense that they are totally unrestricted - most have very severe restrictions on e.g. running servers and many other things.

They mean if you send mail to anyone else - their spam filter will block you for having a residential IP.

Email is a bit less useful when everyone has you auto-blocked for being 'spam'.

I'd forgotten about that part. Yes, having an IP from a pool marked as "dynamic" is enough to get put on many spam blacklists.