Witches can visit the human world, but regular humans cannot visit Alstromeria.
I guess it has to do with witches having an ability to cross worlds that regular humans don't have?
Consider the following:
Witches with contract => extended stay in the human world
Witches without contract => weakening then getting "transported" back to Alstromeria
This is just my guess but witches need some sort of anchor in the human world, lest they get drifted back to where they came from. Hence the need for a contract magic with a human, to keep their existence anchored in this world.