I just dont understand why cant we equip any instrumet we are not proficient with.

We can try with any weapon, armor and shield ... how is flute any different?

Meh. I can kinda see why. Playing an instrument is a bit harder than swinging a sword. You gotta at least have some proficiency or there would be no way to succeed in a Performance check.
Then again. One could say the same about thieves tools. They're not just things you can pick up and just know how to use to pick locks.