It probably wasn't fixed simply because WotC did not put enough care into high level issues. And since very few players ever experience it even once, they let this ugly flaw pass.
What do you mean "probably"?

We've known most if not all designers of D&D from the very beginning haven't wanted to deal with the headaches of trying to balance the "board game" part of high-level D&D math and combat with the narrative story of what being epic-level characters should entail.
While they threw players a bone by setting the game up such that that you could run standard "D&D combat" involving high-level PCs in the exact same fashion you'd run combats in the first couple tiers somewhat okay... I don't think any of them really want anyone to do that. At that level, I think that they think anything PCs should do be almost entirely narratively-focused, because the amount of ridiculous crap those PCs and monsters can do.
High-level play is like spell components or encumbrance... they knew going in that barely anyone ever actually used it, and even if they did those DMs had their own internal ways and means of running it from their historical past. So there was no point in putting together a comprehensive and balanced ruleset for it, because 95% of the DMs who would use it probably would poo-poo the ruleset given and just re-work it to their own specifications.
And quite frankly, I don't really blame them. At some point you reach a point of diminishing returns where trying to placate the DM who doesn't actually want to work on their own game for their own style of playing but just wants everything to work perfectly out of the box... is a fool's errand. Because no matter what they do or come up with, that DM has probably a 99% chance of not liking it anyway.