Transferring copyright has some special consequences if a contribution is made by a company employee as a part of a job, or so I heard. This greatly limits the number of high-quality contributions.
Very, very hard to get corporate lawyers to agree to CLAs, even at companies that encourage contributions to open source.
Yes, and it makes sense, after all you made that part of the code. The same holds true for licenses like the GPL.
Remember the shitstorm about MIR and the CLA that it required? People do not want to sign their rights away and allow a company to sell away their code.