Why does midosuji line constantly reek of mold?

I live in Osaka 14 years ago. Midosuji line used to smell of mold. I am back to Osaka for the 4th time this year and am once again reminded that nothing has changed and every midosuji train car stinks of mold.

Just a rant but like, this isn’t good for commuters. Are people just used to the smell and don’t care or like, what is it? I can understand fixing a whole station (like how Oimachi in Tokyo reeks) is difficult but surely this is a case of just not cleaning the AC units or something.

Anyway, rant over.

by Training-Chain-5572