Because they repeatedly show themselves to be much more competent than any of the other black suited agents?
And it was mentioned that they have some sort of a special spy gene that characterizes them as really good agents. Episodes 102-104, "Totally Busted".
That was hinted at in "Evil Promotion Much?" too (also a three-parter, 76-78 AFAIR).
What I want to know is how it's even legal for them to do so.
It's a world wide spy organization. You think they care about such pesky things as laws? It's not like CIA, KGB, MI6 or anyone else cares about them, apart when they suit them.
It's also possible that the aforementioned "spy gene" will lead to insanity if those with it aren't trained to use their talents for good as soon as possible after it "activates" (by that, I mean the traits become detectable to special WOOHP tech meant for the specific purpose of finding potential agents). The girls' genes activated around the time of the movie, which I believe is a prequel and the Clarks in the sequel activated even younger. (For those interested, this is officially my personal headcannon for this verse.)
Where does the whole thing with Mandy originate from? I mean, she's annoying but she's not that annoying.
Mandy treats the main trio like scum; therefore she's annoying. Plus she grates…a lot.
In the final episode, one of the spies complains about having to put their outfits on by hand as opposed to using their communicators to zap them on. So how did they put their outfits on in seasons 1-2 if not by hand?