I hate doing discovery and system analyst type crap. I'm a Dev, not a technical documenter. I'm not the product owner and I shouldn't be defining requirements for the application. Why does this seem to happen a lot?

Happens to me all the time. Our functional SMEs always say they're too busy to write the specs and the users don't know how or complain to management that its too time consuming to put it in writing. So I usually end up winging it and hope for the best.