Because there's not a fear of people going nuts and killing other people or themselves if their plastic surgery turns out badly, I suspect.
(Note that I'm not commenting on whether the fear is justified - I have a weak-to-moderate belief that it's not, but it's probably more of an issue of risk tolerance in most cases.)
Or because consuming "plastic surgery" is under the surveillance of a professional - a surgeon. Maybe if you had a psychiatrist watching over you every time you consumed a pill, the rules would be different.
There was some talk here about height taxes, but there's a better solution - redefine shortness as a treatable condition and use HGH to cure it. They even got FDA on board with that, at least for 1.2% shortest people.
Unsatisfactory sexual performance became a treatable condition with Viagra. Depression and hyperactivity became treatable conditions with SSRIs. Being ugly is already almost considered a treatable condition, at least one can get that impression from cosmetic surgery ads. Being overweight is universally considered an illness, even though we don't have too many effective treatment options (surgery is unpopular, and effective drugs like fen-phen and ECA are not officially prescribed any more). If we ever figure out how to increase IQ, you can be certain low IQ will be considered a treatable condition too. Almost everything undesirable gets redefined as an illness as soon as an effective way to fix it is developed.
I welcome these changes. Yes, redefining large parts of normal human variability as illness is a lie, but if that's what society needs to work around its taboos against human enhancement, so be it.