Those seem to be 2 different questions and the answer for both are really hard.
why A4 is fixed as 440hz
You might be surpried that A4 is not fixed as 440Hz! Some recordings use A 446Hz instead! (apparently it's a bit more commercial and some thing it has a brighter sound). For more info: https://en.wikipedia.org/wiki/A440_(pitch_standard)
When you have different musicians playing different instruments, you need to tune them all to each other so that the sounds go together nicely. Some instruments (like guitar) are easily tuned, but others like wind instruments are not that easy to adjust. At the end of the day, it makes sense to have everybody agreeing on a single pitch, and A440Hz was the agreed convention. It is just a convention, in the same way that we agree that 1h is made up of 60 minutes instead of 100.
"why divide the scales into 12 pitches even though it is not completely natural?" (since dividing into 12 pitches will make irrational approximate pitch differences, not perfect rational differences..).
The 12 tone scale is the accepted standard in Western music. Historically, other cultures have used scales made up of more than 12 notes (what we call "microtonal scales").
Now, I don't think that you can achieve perfect rational differences. Many phenomena in physics are logarithmic in nature. Even if you chose a different separation (like the 17-interval scale known as 17-TET) you would end up with irrational pitch differences.
At the core of our scale likes the octave, which corresponds to a double frequency. I once heard that given that our inner ear is shaped like a Nautilus, 2 frequencies of the same octave are aligned with each other in the same angle. Unfortunately, I don't have a source for that.
If we take A4 = 440, this means that A5 is 880, and A3 is 220.
Now, that interval is always a different number of Hertz, so no matter in how many intervals you divide it, you will end up with a logarithmic scale of some sort. To visualize this, imagine that we divide the scale in 6 tones instead (a really bad idea, but just humour me).
Visualize the fretboard of a guitar, and now remove every other fret. Still, by having only 6 intervals to an octave, you will end up with logarithmic/irrational intervals.
At the end of the day, it boils down to how our inner ear works, whereby we perceive logarithmic frequency differences to be "equal".