It seems there was a pre-existing agreement to use the GNU GPL with Forgejo, and it seems to me that the GNU AGPL is not compatible with the GNU GPL.
There is more discussion about that around https://codeberg.org/forgejo/discussions/issues/201
I'm assuming that there has been some resistance to using the GNU AGPL with Forgejo (it seems the discussions about licenses has been contentious), and the GNU GPL seems to have been discussed much more than the GNU AGPL. It was probably overwhelmingly likely that we would get Forgejo with the GNU GPL rather than the GNU AGPL. I would have preferred that the GNU AGPL was used instead, but I'm not going to worry about it much since I probably can't change this situation for the better.
Maybe a more appropriate practice is to only engage with a Contributor License Agreement if the repository one contributes to is already available with the GNU AGPL or one is actually in control of some money the person one contracts with will gain due to one's changes. For example:
If I have already fixed a software issue, made it clear what license should be used with my change, and made it available to the public, I wouldn't necessarily be against engaging in a CLA (though I might ask to be paid to do so since I wouldn't normally go out of my way to manually sign things, and I do value my time).
FYI I can navigate to https://blog.hansenpartnership.com/why-microsoft-is-a-good-steward-for-github/ from https://drewdevault.com/2018/10/05/Dont-sign-a-CLA.html (using https://blog.hansenpartnership.com/gpl-as-the-best-licence-governance-and-philosophy/ for an intermediary step), so I'm a little suspicious about the author's thoughts on these matters. I also didn't find any useful information about the GNU Affero General Public License from the same author, and I consider the GNU AGPL to be important based on https://ploum.net/2024-07-01-opensource_sustainability.html and https://lemmy.world/post/16602135