> Assuming the ROI is positive what exactly is holding them back?

The Google problem where every project that is not Search has a much worse ROI.

Yep, and that applies to Valve at two levels because Steam dwarfs the ROI of their games, and their forever-games like Counter Strike dwarf the ROI of any singleplayer game they'd ever be able to make. It's a miracle they even got Alyx out of the door, that was a special case since it was part of their larger VR initiative.