When trying to penetrate armor you either can or you can't. Having 100's of dudes fire on one guy isn't going to improve your chances when your weapons couldn't hurt them in the first place. Unless you are waiting for the off chance the guy hops out of his cockpit to take a wiz or something.
And you would send grunt mechs after them because you didn't have anything else, so you try to compensate with numbers. The gundams in the wing universe were developed by seven geniuses with ridiculous funds and access to rare raw materials. They were almost mythical in nature. And throughout the series you see the opposite faction try //multiple// times to create mobile suits that were of equal capability to the gundams (Vayaete, Mercurius, Epyon, all which were based off the Wing schematics). But yeah, I mean I'm sure they would if they //could//. Why does Ford make Fords and not Ferraris?