Why ETs Conclude Agreements with Earth Factions

As to positive ETs, they respect the individual freedom of other civilizations by default and honour the Prime Directive. Which is the core of what makes them the good ones.

As for the negative ETs, who go by the rule of 'Might Makes Right' or 'Never Give A Sucker An Even Break' there are 3 reasons, why they prefer to make contracts instead of just barging in.

1. Exo-Political: There are a great number of other powerful civilizations and organizations around, most of them positive and if they would openly break the Prime Directive, it could bring them into conflict with the Goodies and create a bigger confrontation. Much like on Earth Super Powers prefer to meddle in the affairs of weaker countries in a clandestine way (Coups, Proxy Wars, etc.). If they have an agreement with a local faction, they can show that to the good ones and say 'See, that's what they want' and it prevents the group that adhere strictly to the Prime Directive to interfere.

2. Practical: The number of every ET faction on this planet is minimal in comparison to the Human Population and the negative ETs present here want to farm earth and its population for one reason or another. So the most elegant solution is to have local farm hands to do the job. Again like on earth Super Powers install local puppet regimes to carry out their agenda. This is one of the deepest and most embarrassing secrets of our so called 'Elites' that they are not more than local wranglers for their masters.

3. Existential: Every advanced civilization is aware of the Universal Law of Cause and Effect, even if they are Dicks. To minimize the backlash of a harmful action they always try to trick or bully their victims into a sort of consent or use proxies to do a dirty job.

Given that Exo-Politics is even more convoluted than Geo-Politics, the situation on Earth is much more complex with the different ET factions involved, but in a nutshell that is what it boils down.