Why not just make OSM "public domain", so no license would be required at all?
A significant part of the article covered that question. It's the same question as why don't we put the Linux kernel in the public domain: The idea is that copyright is valuable property and rather than give it away completely, the owners can trade it for other people giving up some of their copyright. E.g. if I let you distribute my map layer, you have to let everyone distribute yours.
According to the article, some people believe in this with respect to OSM and others believe there's not enough out there to trade for, so it's just not worth the legal trouble.
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