Because eventually we'd have no words with which to discuss things.
This is just not true, though. The English vocabulary is both staggeringly huge to start with, and it can be increased effectively infinitely with creativity, whether in terms of individual words or metaphors.
The basic key of offensiveness in this sense is to make some sort of prejudicial assumption about a group of people that might not be true for many individuals withi in, or to diminish and demean their problems and experiences. There easily exist the words to express ourselves in a way that avoids this. If we're doing something a bit obsessive, there's no need to express it in terms of people with OCD, or autism, or whatever else. There's no need to express an instance of poor co-ordination in terms of people with motor pathologies. There's no need to make any statement based on an assumption gypsies are thieves, black people are dirty, women are hysterical, and so on.
The problem is solely that there are established sayings and usages such that they readily come to mind when we try to express ourselves. We absolutely could change. It would be an effort though, and involve slip-ups, and also needing to be open and consider perspectives that something we did not think offensive in fact can be.
I suspect resistance to doing so comes down to a) the effort involved, b) unwillingness to admit fault compounded by c) backlash against criticism, d) ideology (sociopolitical stances), and more unpleasantly e) belief prejudices are true, f) enjoying being offensive. I'm not sure any of those factors inherently merit great respect.