It’s an arbitrary thing. We use base ten, meaning we use ten digits, 0 - 9. When we reach 9, we roll over to 1 ten and 0 ones, or 10. Like another user pointed out, this may be a result of our number of fingers. But we could use base twelve. Or eight. Or seven.

So the answer to your question is: because some old dead dudes decided to start, and we kept it going.