Can you think of a single instance in which code written by an individual contributor not bound by an employment contract was prevented from being incorporated into a project that required copyright be assigned to a central entity?
I have read multiple accounts of open-source contributors who have successfully negotiated when being hired that copyright to any code written on their own time on the own hardware will remain with them (so as not to complicate their contributions to open-source projects).
Intel and ARM both collaborate on GCC and Linux all the time while being fierce competitors. Google, Canonical, Red Hat, etc all contribute to Linux while being fierce competitors.
But the only companies that contribute to Android and ChromeOS are device partners, and the partner contributions are far more limited in scope.
Wikipedia has to worry about the same thing, BTW: contributors who upload text whose copyright is owned by some entity who never consented to the upload, and they have policies in procedures in place to correct it when it happens.
It's not like CLA's are the only obstacle to employees contributing to the open-source projects. In fact, you might say that the main purpose of a CLA is to surface copyright-ownership problems early, when the cost of correcting them is lower.
GCC itself has definitely been inhibited by the copyright assignment. There have been a few different ports that were not upstreamed specifically because of the CLA. AVR32 and C6000 come to mind off the top of my head.