The short answer is: in order to produce the correct interval sounds of the chromatic scale in higher registers.
The longer answer has to do with physics. Essentially, musical tones are based on logarithmic ratios instead of linear, equal steps. So, relatively lower sounds have wide, long, slow frequencies; conversely relatively high sounds have narrow, short, fast frequencies. And because the distance between the notes is logarithmic that distance results in a requirement to shorten each fret spacing roughly 5% going up in pitch.
This is also the reason the strings of a piano are long & thick on the bass side & gradually get shorter & thinner on the treble side.
There are several sites on the internet that explain it in more detail & some with images.
https://www.liutaiomottola.com/formulae/fret.htm
Wikipedia also has some good entries on musical notes & physics.
Hope that helps!
