because it doesn't do anything useful. mempool settings are incredibly niche for non-mining nodes, unless you are optimizing memory, but then you would just use the total mempool size setting. I've been running a node since 2010 and I never once needed to tweak this, only ever mempool memory usage. non-useful code is dead code and is a maintenance burden. non-useful code is also confusing to users who think tweaking the knob will actually do something.