Why do RNAV airways exist? I assume there's some stupidly obvious reason that I'm missing, but why bother creating RNAV airways when I thought the entire point of RNAV was not needing to fly along a suboptimal preset path?

I can see the need for waypoints, especially around the corners of restricted airspace that you'd need to dodge rather than just flying direct everywhere, but I'd think an RNAV flight plan would just be a handful of such critical waypoints--no "airways" needed.
RNAV freed aircraft from the airways that were already in place that may have zig zagged from VOR to VOR, but with the limitations of the ATC system based on human controllers, it is still desirable to keep most aircraft on a "road network" so to speak to make it easy to manage separation, so RNAV airways were created between major centers that provide straighter roads between cities you might say.

Try to imagine thousands of flights going hither and yon all on their own random tracks, instead of being fed along specific airways, without a bank of supercomputers to manage the separation, and you can see the problem that the RNAV airways solved.