I used to think that copyright is always assigned to the creator, like in Germany, and it appears that I was wrong: according to Wikipedia, at least English law actually defaults (no contract clause needed!) to assigning your copyright to your employer if the contribution was done as part of work for hire. This was a surprise to me but it explained why some OSS projects, like ones by Adobe, require a CLA: many people use their libraries at work, and if someone like that contributes a fix Adobe’s lawyers justifiedly would not want part of their code to be owned by another company.
It is a sad side-effect that assigning away your rights with a CLA to some company also enables some shady behavior[0], but it seems that the possible intent to “to place a rug under the project, so that they can pull at the first sign of a bad quarter” co-exists with a more reasonable desire not to have parts of the codebase that you started and mostly maintain at your own cost owned by a potentially hostile entity.
That said, it’s sad that DCOs are not used instead[1]. IIUC, DCO basically makes it clear that the contributor is the one owning the copyright, eliminating the above issue without enabling the rug-pulling.
[0] https://drewdevault.com/2023/07/04/Dont-sign-a-CLA-2.html