For instance, everyone tells you not to optimize CPU time outside of hotspots, but with memory usage even a brief peak of memory usage in cold code is really bad for system performance because it'll kick everything else out of memory.
Given how incredibly successful it's been, it's conceivable the suits decided the opportunity cost of investing man-hours to fix the issue was too high, and that effort would be better spent elsewhere.
We're talking about an issue that has been loudly complained about for 7 years (and I am evidence that it makes people play the game much less often than they would like to) that some person without access to the source code was able to identify and fix relatively quickly, it would be surprising if it took an FTE 1 week to find this with access to the source code.
This is one of the most profitable games ever, they could hire an entire team to track this down for half a year and it wouldn't even be noticeable on their balance sheet. And I would bet money that it would have increased their active player base (which they care about because of the micro-transactions) in a noticeable way, as long as players were aware of the improvement so they would try it again.