Not requiring a CLA doesn't necessarily mean you just didn't think of it... you can not require a CLA on purpose. To spread ownership around to enough people to make it much, much harder to change the license. To make the license as sticky as possible.
I think a big reason that CLAs are less common nowadays is Github's TOS which explicitly require you to acknowledge that you contribute under the same license:
> Whenever you add Content to a repository containing notice of a license, you license that Content under the same terms, and you agree that you have the right to license that Content under those terms. If you have a separate agreement to license that Content under different terms, such as a contributor license agreement, that agreement will supersede.
While one can argue that even in the absence of that notice, that might be how it works, there is no real legal agreement to that. You could legally walk back on your contribution and claim that you never gave a license to it. I have seen that happen once indirectly when a contributor informed me that the company that they worked for did not allow the person to contribute.
Having a CLA like this would seem to cover your ass better legaly-speaking, but I'm not sure if either case has been tested in court and so am not sure if it really does help or not.
I've never personally seen a CLA where you weren't signing away ownership, where you were just stipulating that your contributions were under the same license as the rest of the code. Do you know of any cases of this? I'd be curious to see them.
