In sales it is uniquely easy to identify your contribution since it's literally measured in dollars coming in.
There's no way to quantify that for feature implemented/bug fixed, it would devolve into endless politics.
> how come sales people are unable to do things for a fixed monthly salary?
Sure they are able, but no good sales person is interested. Presumably you'd want to hire the good ones. You can certainly staff a mediocre sales team on a fixed salary, they just won't do much selling.