Because all nine of the “teen” numbers aren’t pronounced onety?
how about a bunch of numbers aren’t pronounced like that twoty two? threety three? fivety five?
BUT WHY AREN’T THEY
If I may… The words for “eleven” and “twelve” are likely holdovers from pre-decimal counting systems that were not base-ten. That is, they get their own special words because pre-decimal system, they were “single digits.”
The duodecimal (also “dozenal”) system (base-twelve) is still used throughout the english language for quantities and measurements of things: We buy a dozen eggs, not ten. There are 12 inches in a foot. When you buy in bulk, you buy a gross = 144 (12 x 12) which is the duodecimal “100”
The earliest counting systems were sexagesimal (base-60), and we still use the sexagesimal system for time, measuring angles (360 degrees = 60 x 6), and geographic coordinates.
So WHY, 12 and 60? Both are highly composite numbers. Both also have an exceptional number of prime factors!