Why are a piano’s black keys arranged in groups of three and two?

Question

Why are a piano’s black keys arranged in groups of three and two?

In: 44

Answer 1

C – C# – D – D# – E – F – F# – G – G# -A – A# -B – C

We agreed that this is default c major scale.
To be able to easily find them on keyboard, we decided to make ones with # different color.

So, its just a matter of agreement