I suspect that the core engine programmers moved onto other projects long ago, leaving GTA:O running with mostly artists and scenario designers to produce more DLC.

This bug wouldn't present in the first couple years with the limited amount of DLC, so by the time it got ridiculous there wasn't anyone left with the confidence to profile the game and optimize it. A junior dev could fix this, but would probably assume that slow loads are a deep complex engine problem that they won't be able to fix.

Alternatively, management would declare that there's too much risk doing more technical engine work, and not sign off on any proposed "minor" optimizations because it's too risky.