Okay, I get what you are saying. Two simultaneous notes a minor 2nd apart sound like two distinct notes. But you don't have to narrow the gap by much more before it sounds like a single note.

A very small gap gives a subtle chorus effect, a little wider takes one into honky-tonk piano territory, and wider still perhaps more like a bell, but still sounding like one note.

But, as you say this doesn't negate the audibility of microtonal inflections in a melodic line.

However, pitch steps smaller that those of 12tet can still provide new chords. For example, in 24tet the triad with a third half-way between minor and major is a distinct and interesting sound. But yes, in 24tet two consecutive pitches sounded together sounds more like a single note with an interesting timbre.

Yes I do not negate that microtonal has interesting effects. The question was why there a 12 tones in western music. And the answer is that minor 2nd is the smallest interval you can separate when played together. I should have been more precise in my initial comment.