Because the model of private capital using open source to make profits is a failure state that we need to get away from. There's no reason why the government can't sponsor open source projects, something tells me the vast majority of open source devs wouldn't mind a system where grants can be reward to projects that the public finds valuable.
That would be much more sustainable than VC rat fucking the commons to make a buck while suckering in devs that were once good community stewards into dry husks that are only formed to generate profit.
Ideally, the corporations that get immeasurable benefits from open source are a better source for the money. There are multiple ways this can play out, direct payments, putting employees on the project, or contributing their own projects to benefit others.
Ok but those government grants don't really exist today and what you're arguing for is zero sustainability for open source projects. This is certain to lead to the death of open source - there's not even the reputational pay-off any more if the only real consumer is AI.