They've probably already "been here, done that". Having vastly more intelligent and technologically advanced aliens watching us human beings go about our daily routines would probably be about as interesting to them as you or me watching some cows in a field going about their daily routines. The aliens probably visited us, took some notes, and then moved on to explore other worlds.
But the farmer has interests in the cows other than looking at it.
A better anology might be a native creature, like the buffalo. Even if we couldn't eat them, we'd still have had an interest in the land they were on. And even if aliens were good-hearted enough to let us have Earth, why not use Mars and Venus?