They have to be on the trade list because you can demand them if you are winning a war and offer them peace for planets. As for being "easy" to build an AI to assess trades, spoken like a person who's never designed a game. Influence is just one factor to consider:

What is the planet morale? Maybe you've built no morale structures on it and it's ready to rebel?
Mayby the planet you want would open a new section of the galaxy to your colonization, which means the quality of the planet you want means less than it's location?
Or maybe the planet you want would block off the AI's access to further colonization?

There are many reasons you could want a planet that would be difficult for the AI to assess in real time.