1. They are lonely. Speaking with other Teruvar is hard on the creatures of their interiors, and they can quite easily chat with interior people.
2. They are curious. The people in their interiors are mayfly like. Evanescent. Their lives are over in a blink. How can they sustain themselves?
3. They wish to acquire some technology for long range reasons: * Radio - to lift their loneliness without bothering their internal creatures. They can talk to anyone with a radio, instantaneously. * FTL - to jump to other systems. To travel. To see new things. To spread their seed in new systems. * Fusion Reaction Jets - for the same reasons as FTL. It will just take longer, and that is no problem to a creature with tens or hundreds of millions of years to live.
4. They want the short-life tiny ones to spread their seed around other stars for them, carrying it as cargo.
5. They eventually want to make Humans and Jeshen part of their default internal creatures, each seed carrying the genetic blueprint of them internally. Eventually these tiny people will be inside every Teruvar, everywhere.
6. People are entertaining, and the current internal creatures are boring. They enjoy the antics of the newcomers, and love to meddle.
7. They want to add the intelligence and sapience of these people to their default animals, by stripping and analyzing their genetic makeup, and incorporating analogous genes into some of their internal creatures.
8. They like being worshiped like gods.
Any and all of these reasons could apply to any given one of the Seven.