Why did we choose to put a note between c and d but have b and c be a semitone away? Why couldn't we have divided the 12 tones as a a♯ b b♯ c c♯ d d♯ e e♯ f f♯ as that would be much simpler to understand?
The reason for this is that the note names more or less came first, and then some chromatic alterations were added, eventually filling up the chromatic scale. The way note names are chosen is consistent with the melodic concepts of traditional west European music. The notes A-B-C-D-E-F-G form a minor scale and thus simply by shifting also a major scale.
Instead if we chose note names symmetrically then suddenly the notes A-B-C-D-E-F would form a whole tone scale. Then suddenly C major would not be simply C-D-E-F-G-A-B-C, but it would be B#-C#-D#-E-F-C-B-B# (note we have to double a note name as we have one less note name).
Now, if traditional European music was in any way centered around whole tone scales this would have been a viable option, but this is not the case.
Just imagine: Your child in music school learns the note names of "Ah! vous dirai-je maman" (twinkle, twinkle, little star, whatever), and instead of C-C-G-G-A-A-G F-F-E-E-D-D-C you go C-C-F#-F#-A#-A#-F# E#-E#-E-E-D-D-C. And now tell me which of these would be more simple to understand?