That’s because they tried integration with the rest of their SW ecosystem.
If they just did motherboards with cpu/ram/pcie and let other OEMs build complete systems the story would be very different
They tried it with various open systems that allowed licensed powerpc clones of their systems and they ended it because it reduced their profits as people bought the clones instead of mac hardware. I don't see them redoing that anytime soon.
That still doesn’t overcome the issue that they are selling every single one they can already. They have no need for additional chip sales.
All the capacity at all the fabs is booked up for years. They can’t make more if they want to.
So any hypothetical sales of OEM style guts would only cut into the number of Macs they can sell to existing customers. And there’s no way they would ever do that.