And why is this? Why not just name the notes A-J. I mean, the spacing between frets is the same between notes A and A# as between B and C. I'm so confused.
Sorry for the stupid question. I have absolutely no musical experience (obviously) . . .
There is no satisfying practical answer for this question. It goes back to the earlier days of European music.
My understanding is that the notation goes back to the use of a single scale, which was what we now call the aeolian mode which predates the ionian mode (major scale) as the most widely used scale in European music, with the use of A as the first letter name being a vestige of this. The need for describing other notes and then other starting points led to the use of modifiers for the 7 notes in the primary scales. Later this contributed to the development of equal temperament which helped promotes using more keys because the intervals are consistent.
If it had been designed from the get go for equal temperament without favoring any particular style of harmony, yes, it would have made more sense to use more letters or to use numbers or some other set of identifiers which might be more suited to looping around in each octave for intervallic calculation.