Alright, this is a complicated question, but I'll try to boil it down.
Remember when we said that simpler ratios sound more harmonious? Turns out that actually using them to make music, is not so simple.
This is not the place to get into the details of it, but since, as we know, the circle repeats the same at every Octave (i.e. every multiple of two), we have to figure out a way to divide the space between one (tonic) and two (octave) into harmonious steps that we can use to make music.
However, we do not only want the relationship between those sounds and the tonic to be harmonious, we also want them to be harmonious to each other. And, ideally, in Western music, we also want to take any of those sounds and make it the tonic (i.e. the center of our musical piece).
Oh! And we also want instruments that are really impractical (or impossible) to retune on the fly: such as the piano, the organ or even just a flute.
To satisfy all these requirements is mathematically impossible (trust me!), and has been the crux of composers, musicians and instrument builders for centuries, and many systems with different compromises have been devised.
At some point, in the West (actually the Chinese came first, but that's a story for another time…), we decided to compromise this way: take the octave and divide it into twelve equal pieces; tada! Now all the relationships between the notes are equal!
“But wait!” I hear you say “Doesn't this mean that no note is in a simple harmonious relationship with another anymore?”
But we decided it was good enough, and that if we start stacking a lot of notes on top of each other and make a huge noise, we hoped that no one would notice!