I think it has to do with supply vs demand issue - like any other fields. Even at the PhD level, there's not enough academic position for every graduate, so low-paying postdoctoral positions were created for that purpose. But it's also true that things I can do with biology degree are many - writing is one example, haha.

Anyways, all the best to your geoscience degree. If I'm wrong, climate change is going to be even more important in the future.