Because they're a lot less capable than these companies are telling us they are.
Don't get me wrong, you can frequently get some excellent results with them... but you can also get some really shit ones.
So not only does the bulk of this work require someone to do all the prompts, they also need to thoroughly check the work afterwards, meaning you're not really gaining much, if anything at all.
Sooner or later, the venture capital propping up AI will realise that these enormous savings from laying people off en-masse isn't going to materialise, and they'll want their money back. The market correction will be huge.
There's nothing wrong with giving money to FOSS projects.
In fact, a major issue with the open source world is users never donating.
I've never used Zorin and don't intend to, but the existence of optional paid software isn't why.