Why is it normal to say 'getting old' or 'getting fat' etc but usual to say 'going bald', 'going deaf' etc? As far as I can see all these are progressive conditions none of which is welcome.
It's all part of the wonderful richness of the English language. Both verbs, get and go, can mean 'become' as well as a raft of other meanings. Over time, each has settled into idiomatic usage in slightly different ways; hence, getting fat and going bald. I suppose it might as easily have been going fat and getting bald. Indeed, going bald often IS used.