-----
The reason the systems are made this way is also because these "Gods of Transmigration" get a share of the EXP for themselves, gaining Soul Power, and further strengthening themselves. The higher level their champions, the stronger they can grow.
Though seeing how selfish they actually were, I doubt they do this without personal gain in mind, after all, there's no way they would just do this because they felt bad for the people or something, they specifically send heroes to worlds to harvest their Souls through their hero's Systems.
At the same time, they "solve" problems there that are making their Soul Reincarnation Cycles slower. Or so I've learned from Alice as she had told me about them the more she regains her memories or feel that they had become clearer.