Why do RNAV airways exist?
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.