Well, the reason why they get hired is because the FW corp leaders just don't care.

You see, when you start investigating the lore revolving around the whole Factional Warfare, also known as the Empyrean War, things get interesting. Firstly, the goal of FW isn't to actually make it winnable. Even if you've conquered every single planet and system, nothing special happens and a few months later the tables have flipped again. It's been long established between capsuleers that the whole Empyrean War is just a facade, a stage to keep all the empires in check, preventing an actual war from happening.

And really, the general of Federal Marines has little to nothing to do with the capsuleers and dusters who participate in FW, nor do the leaders of any other empire army or navy. The actual fighting is done by proxy militia corporations, that are completely separate from the actual Empire armies and navies they represent, they just set up the contracts and system for us to fight in.

Well, there was really only one lore response to this, which I studied and looked up for a few hours after reading your question. There could have been more participants over the weekend, but there aren't too many people that are interested enough in the lore to look this up and study it.