Because the monetization wouldn't work; anyone can just recompile the FOSS code and legally distribute a free build and users & PC manufacturers would flock to that.

That's already how it is, and yet the majority choose to pay for the convenience of two proprietary systems. We still haven't seen a FOSS offering that's well-suited for the typical end user IMHO.

I mean, Elementary OS exists... but it doesn't seem to be very popular for some reason.