Hi, this isn't a history question per se, but someone here may be able to help you. However, this question might be better placed in /r/learnmath or /r/AskScience (mathematics category)

This is an effect of the fact that we use base ten numbering system. This developed likely because we have ten fingers and ten toes, making ten a good number to group by. But anyway, the way it works is, we have the numbers 0-9. That's it. Ten is just a 1 and a 0. It doesn't have its own symbol. So, 10 is the decimal (base ten) representation of, well, ten.

To best understand how and why decimal notation of numbers works, it is useful to examine another system of representing numbers. You don't have to represent ten as 10. If we used binary to represent ten, its 1010. in hexidecimal (base 16) ten is A. Hexidecimal uses 0-9, then A-F, where A is ten and F is fifteen.

You know how when you're in school as a young child, and they teach you about the one's place, the ten's place, the hundreds place, and so on? That's because 10^3 is 100. Base ten. 3 places. One hundred. When you write out one hundred in numerals, what you're really saying is that you have one of these clumps of one hundred, zero clumps of ten, and zero clumps of one.

Let's go back to binary. Let's take the number one hundred, and represent it in binary. It's 1100100. How does that work? Well, instead of needing three digits to represent one hundred, with a ones place (ten^zeroth), tens place (ten^first), and a hundreds place (ten^second), we need instead need a ones place (two^zeroth), a twos place (two^first), a fours place (two^second), an eights place (two^third), a sixteens place (two^fourth), a thirty-twos place (two^fifth), and a sixty-fours place (two^sixth). If that seems confusing, think about what "100" actually represents.

It's 1(10^2) + 0(10^1) + 0(10^1)

Punch exactly that into a calculator, and it spits out 100.

With binary, you're basically just adding powers of two until you get the desired number.

1(2^6) + 1(2^5) +0(2^4) + 0(2^3) + 1(2^2) + 0(2^1) + 0(2^0) =

64 + 32 + 0 + 0 + 4 + 0 + 0 =

96 + 4 = 100

There are an infinite number of ways to represent numbers. Base ten is the one we use in our day to day lives, but at the very most basic level computers only understand ones and zeros. Base eight and base sixteen are used to more efficiently represent binary numbers sometimes, because, since eight and sixteen are powers of two, its easier to go between them than it is to jump to base ten, for example.

If any of that was unclear, I can try to clean it up a bit. And now, for a wonderful joke:

There are only 10 kinds of people in the world. Those who understand binary, and those who don't.

And those who weren't expecting trinary!

Edit: There's a great book, The Universal History of Numbers that is a great resource for things like this!