Why not lineal feedback, instead of rotary encoders?

I have been looking around the groups, www, commercial vendor sites,
etc and have noticed that positional location feedback is always
determined by an encoder mounted to the screw or motor. I can't
figure out why the actual position of the object (table, workpiece,
tool, or whatever)is not determined by a linear transducer, eg a
scale not unlike one used with a dro.
Wouldn't this be more precise. We are intending mostly to measure
linear - planar, and not a rotating object's final position afterall.

And "final position" doesn't have to mean stationary, it means final
before the next reading, position update, polling, and so on.

It is the object's location that we are currently interested in, not
the rotation of or position of the screw or screw motor spindle.

Measuring the screw's location (ie. number of turns or part thereof)
is not quite the same as the actual location of the work - due to
sloppiness in anything, the screw, bearings, screwnuts, collars,
retainers, gibs, gears, belt drives, transmissions, friction, etc .

A positional feedback system derived from measurement upon the actual
article we are forcibly locating would then be somewhat immune to the
abovementioned sloppiness, wouldnt it?

So why isnt it done?