well, how about because it's inaccurate? It's not just basses, but guitars, ukes, banjos, anything with frets. there is always a trade off because of the strings being in a straight line or fanned or whatever. Not every note fretted will ring to true pitch...the squiggly fret addresses that problem head on.
It's not really that big of a problem. In fact the ONLY stringed instruments that can play in tune in every key are fretless ones... yes, including pianos.
Musical pitches are most harmonious when their frequencies are ni specific ratios to each other. If you tune a piano to one key signature, let's say "C", those ratios are not correct for other key signatures. For that reason pianos and fretted instruments are tuned to what is called "even-tempered" tuning which is a "best-compromise" tuning so that you can play in different keys and not sound bad.
IOW a "C" in the key of "C" is, strictly speaking, a different frequency than a "C" in the key of "F". We are listening to "out of tune" notes ALL THE TIME.
Now if you really want to talk about pitch accuracy, on fretted stringed instruments, the frets are typically positioned at the exact locations specified by the equal-tempered definition. But when you fret the string it not only shortens the effective sounding length but, to a lesser or more degree, you are increasing the tension which changes the pitch from what it should be!!! The amount of tension change is different for different depending on how far up the neck you fret, string material (differen mfrs use different formulas) AND string gauges!!
My point?
1. We are listening to out of tune notes all the time. People fretting (sorry) over this to the nth degree are chasing windmills.
2. The ONLY way to play a guitar or bass perfectly in tune up and down the fretboard is to NOT HAVE FRETS.
3. Since we are listening to out of tune notes all the time, our auditory system is pretty accomodating of small pitch errors. Otherwise, we could NEVER enjoy music. It's a small miracle that we do.
