Wow, that repo man is so OP I think they're better off killing the star gods and taking over their empire.
The idea is that instead of outright losing when you have two consecutive months in the red (or when you fail to repay one of their loans), they come in endless waves to 'Repo' your bases and everything inside (including your Gals/personnel as to-be slaves);
dead or alive, you're coming with them. Basically a soft lose condition until you either are crushed, or settle up and get out of the red.
More broadly lorewise, yeah, they _could_ try to leverage their Repo Men in conquest, but if they did, the Star Gods would take note, and not even they would stand a chance against the combined might of the Mercenaries and Star God armadas; the Empire is vast and even if somehow ultimately successful on Earth at opposing their rule despite their lack of air/space power, they have no way of dealing with the crushing retaliation that would be sure to come with such open rebellion (which might include the planet's wholesale destruction if they were deemed enough of a threat), or at the very least, that's how they figure. Better to keep a low profile and make shit tons of credits instead; you'll live longer.