Some French speaking countries use a more logic term for higher numbers. In Belgium and Switzerland 99 would be nonante-neuf which is ninety-nine.

My uneducated guess is that at the time, it was easier to keep the base 20 to visualize a higher number. Just like we say "a few dozen eggs" to get an overall sense of the volume, it made more sense to keep it that way instead of a more arithmetic approach. Not sure why it never evolved, tho.