And your fork has the advantage that it can take code from the upstream tree (as per licence) but the upstream tree can't take your code without dropping their copyright assignment requirement (or maintaining a seperate "copyright pure" fork for their closed releases).

If there are significant engineering resources outside upstream then that leaves upstream at a disadvantage and at risk of not being considered upstream any more.

Copyright assignment only makes sense for either side when those requiring it are (and will be) doing the vast majority of the work.

There may be other considerations (such as ease of relicencing to a different open license rather than releaseing a closed version) that make copyright assignment attractive in some cases but some licenses accomodate for that fairly well in themselves.