The universe is a garden and aliens that visit earth come to make sure life continues, they may even find earth like planets or other planets they find capable of hosting life and planting it themselves. Not as an experment but more also as a continuation of the thing we all have in common.

Some of the organic "craft" you've been seeing come from other parts of the universe

The universe is alive, the milky way is alive, earth is alive. Entities within entities that host what we all have in common