Because.
That is how the English language has worked for years and years and years and makes the most sense.
No. 1 is Number One, or number one.
It is spell one hundred and ten.
If you're meaning how you spell that number it's ten point one.
One.
one
The number 2000 is written "two thousand."
It is - one millionth
one thousand
"Uno" is Spanish for the number "one". The number one in German is "ein".
1-um
The number 171 is "one hundred seventy-one."
The number is one thousand, one hundred (and no thousandths).