Something I wonder, doesn't require copyright assignment reduce the number of contributions? What is the philosophical standpoint behind requiring copyright ass

Something I wonder, doesn't require copyright assignment reduce the number of contributions? What is the philosophical standpoint behind requiring copyright assignment, wrt. being part of the OSS community? A copyright assignment gives power to the project owner to end up relicensing all those contributions as proprietary, effectively extracting free labor from contributions that were originally written under the implicit social contract of being part of the OSS community and all the Good Things that come with it.