I never understood why the piano keyboard isn't regular. It forces players to remember different positions for the same chord transposed to start at different notes. Like why do I have to remember the shape for C major and D major chords? It should be the same shape just starting at C vs D. It's not even that hard to fix. There's 12 semitones in an octave. Just make it 6 white 6 black keys.
The white keys form a sequence of notes (frequencies) that is known as the diatonic scale. It's the foundation underlying all popular western music. It is not random or arbitrary, it has some nice dual mathematical and musical properties: intervals between the notes in the scale have special frequency ratios that sound pleasing to the ear (read Helmholtz's "On the sensations of tone" for a fascinating physically-base…
Vast majority of fretted instruments since the death of the lute are untempered.
Edit: Which is not to suggest that lutes were tempered. Lutes and other tied fret instruments allow for unequal fret spacing so you can temper one string at the cost of more notes being more off from the temperament on other strings, or the frets being at an angle so you could find a bit of a compromise. But often they were EDO or in the ancient tradition of fretted instruments, close enough for rock and roll.