> If you are a civ who is able to do it, surely you can grow whatever food you need at home and have advanced AI/robotics that can provide labour.

That's about as reasonable an argument as "if you drive a Tesla, surely you can afford to donate to my cause". Maybe they are way over-invested in their FTL technology and really have no choice but to look for external labour. Maybe they painted themselves into a corner with the FTL tech that can get them here, but they need our labour to enable their drives to restart for the trip back. Or maybe whatever reasoning they have is so _alien_ to us that we simply can not comprehend it.