Also, 9900 was much easier to say... how do you pronounce 10900? "Ten-thousand-nine-hundred"? Feels tortured. "Ten-nine-hundred"? Yeesh.
Compare 10th gen i9 to i7:
https://ark.intel.com/content/www/us/en/ark/products/series/...
https://ark.intel.com/content/www/us/en/ark/products/series/...
Eight oh eight eight? Eighty eighty eight?