I'd think it's also much easier to spin up a (in some area) slightly better clone and eat into their revenue.

This is part of it for sure. It is also true that many open source business depended on it not being worth the trouble to figure out the hosting setup, ops etc, and the code. Typical open source businesses also make a practice of running a few features back on the public repo.

Now I can take an open source repo and just add the missing features, fix the bugs, deploy in a few hours. The value of integration and bug-fixing when the code is available is now a single capable dev for a few hours, instead of an internal team. The calculus is completely different.