Because then they'll have to compete with Linux, which opens them up to someone else making a cheaper computer that does the same stuff.
I mean, I'm typing this on a junky Raptor Lake laptop I found on eBay for $220. Would I buy a Neo or MBP to replace it just to run the same software stack? Probably not.
If you believe OS X is the best environment and Apple Silicon will remain the best hardware in perpetuity, it seems silly for Apple not to allow some experimentation. Apple executives, to be blunt, know better. Lean times are always ahead.