Because the code still has to be actively maintained perpetually - internal APIs change over time, security risks have to be mitigated, some one (or some teams) have to have responsibility over this feature - and Google has decided it's not moving the needle enough to be worth it.
While technically true in this age of social distancing and increasingly online everything it probably should.
Youtube is essentially a Monopsony for monetized Internet video. So this will make many previously accessible channels inaccessible to the deaf and hard of hearing.