Because we decided on it. We use a base-10 system with ten digits (0-9), and we prefix digits to indicate powers of ten (so the rightmost digit is the ones, the next one to the left is tens, then hundreds, then thousands etc)

We can do the same with any base. You may have heard that computers and computer programmers often use binary. That's base 2 (where we use the digits 0 and 1, and the rightmost digit is ones, the next is twos, then fours, then eights and so on).

Hexadecimal is also commonly used with computers. That's base-16. So we use the digits 0-9 plus the letters a-f to mean "ten" through "fifteen". So the rightmost digit is, as always, the ones, then we have the sixteens, then the 256's and so on.

The general system works with any number. We've just decided to settle on 10 for our everyday number needs.