Because nobody ever really tested the system that far, so nobody ever realized that the scaling just doesn't work because the numbers become so big that every action is either so minor as to be irrelevant or so major as to be instant win.
Originally it was not that important because very few people long enough to get that far, and the few that did simply did not care. That held up through AD&D 2nd edition.

When D20 came around they tried making plans for it but they still did not playtest much beyond 10th level, and by the time they got that far they had added so much splat that optimization made it even worse, and there was nothing to do but shrug and pretend it didn't exist even though everyone was fully aware of it.

When they tried "fixing" it after D20, they not only messed up the math, they gutted the things that made the classes distinct to the point that they made "refluffing" a default element.

I expect the same problem still exists with the current version of the rules.