It's not a matter of "running more cables", and not just a matter of cost. They have to complete their IPTV migration, which is a massive project, and has been in progress for years now. They also have to upgrade hardware at every single node, which is a multi-year project. With that also comes brand new noise mitigation problems.

Yes, you can configure it to use more channels for upstream, but only if you cut off television to all of those paying customers, likely losing a lot of subscribers, breaking carrier agreement contracts, lose the advertising dollars, and go out of business before you even get there. You also have to swap out all of their cable boxes, and leave whoever purchased their own box hanging.

As far as I know, no ISP in North America is currently using any DOCSIS 3.1 features for upload, and they're barely taking advantage of the features for download. Moving to OFDMA is the big barrier to opening upload speeds. The protocol supports it on paper, but turning paper into massive co-existing television and ISP infrastructure is not trivial.