A mechanical aperture ring has a lot of mechanical components and thus costs money to design and produce.
It also takes up space and most Nikkors have different internal designs to overcome this due to the size and position of the rear elements and how they move.
I guess some people still prefer a typewriter to a keyboard
Really none of the new lenses performs well with extension due to their highly optimized lens designs with floating elements.
One exception is the New Noct-Nikkor 58mm 1.4 AFS that performs quite well on a short extension ring... it has no floating elements
So one could argue that a new fully electronic extension ring should be available from Nikon, but the marked is probably way too small...
Rip out the guts of a TC-E if you like - it's a nice DIY job, works perfectly.