Aside from the cost,

Aside from the cost, is there a technical reason that ball screws aren’t used for higher precision instead of threaded rod?

Mass, and in the case of the Z axis, weight. 3d printers operate best when they can accelerate very rapidly.

I would add noise as well. 6mm rod with a pair brass nuts and a spring is practically silent with no backlash.

Well, mainly it’s cost and because we’ve always done it that way.

Although not yet in full production, the QU-BD RPM uses ball screws because of the torque needed for milling that it also can do. Even so, it can do printing (not just travel) at 150mm/s.The revoXL that they are making is doing repeatable prints at 500mm/s in ABS and using threaded rod. I’ve included a couple links.
http://www.qu-bd.com/general/high-speed-printing-is-a-reality/

Both are under $2000 but you’d have to ask them where they are getting their ball screws.

A few years ago I heard that advances in materials and manufacturing meant that you could buy acme screws with precision and durability comparable to good ground ballscrews. I believe that the only restriction was that they were only available in relatively small sizes and lengths - too small for industrial CNC machines, but certainly large enough for any RepRap.

High precision machine tools have mostly moved from servos and ballscrews to linear motors. Glass scales (or interferometry) feedback is necessary, since there is no spinning shaft for encoder feedback - but that class of machine already had them, so it wasn’t an added cost.

It’s possible for an individual to put together a working linear motor; the same can’t be said for a servo motor and/or ballscrew.