Because it prevents a choice of more liberal licensing later.
Ogre3D was able to successfully transition from LGPL to MID/BSD because of contributor agreements.
The Apache Software Foundation, Free Software Foundation, and others all require contributor agreements for very good reasons.
If your projects starts out with very liberal licensing, it's not necessary strictly speaking (unless you're concerned about patents). But that's the tradeoff.
It might also discourage contributors who don't want a transition to a different license. After all, contributor agreements also give the power to go closed source later on.
Like I said, tradeoffs. LLVM for example doesn't require copyright assignment, but does require a contributor agreement for patent purposes for significant contributions.
Contributor agreements are a necessary mechanism to protect the project and ensure its future when dealing with patents or preparing for the potential of relicensing.
Yeah, but the Apache Foundation already has a permissive license, and nobody expects the FSF to re-license a project, except maybe to a new version of the same.
By choosing the (A)GPL but forcing contributors to grant relicensing rights, you've excluded both those who consider copyleft licenses non-free, and those who consider copyleft licensing an important requirement.
Did you miss the part where I mentioned patent agreements which is another good reason why Contributor Agreements are important?
And while the Apache Foundation already has a "permissive license" it's not quite as permissive as BSD / etc.
And if the Apache Foundation wants to relicense, to say, a future version of their license, they're still going to potentially need that contributor agreement.
Ogre3D was able to successfully transition from LGPL to MID/BSD because of contributor agreements.
The Apache Software Foundation, Free Software Foundation, and others all require contributor agreements for very good reasons.
If your projects starts out with very liberal licensing, it's not necessary strictly speaking (unless you're concerned about patents). But that's the tradeoff.