Why is self injuring considered more unacceptable then using extreme methods to lose weight?

It is because society is overtly accepting of the end result of that extremely dangerous weight loss; in other words, it is seen as marginally beneficially, in a superficial way. Self injury doesn’t have an outward effect other than signaling that there is a problem within oneself, but excessive use of weight loss products, such a diuretics, is merely a social taboo that people look past, mainly due to their own inner insecurities.
It doesn’t really make all too much sense, and that’s the problem, challenging societies values is a very illogical thing.