Because according to the Gitlab comment you've taken that excerpt from, companies are so allergic to the GPL that they will not only avoid using the GPL version of libxml2, no. They will maintain a MIT fork of libxml2 out of spite, even though they've been actively running away from their responsibility to do so.

Let them maintain it. We can take their permissive code and add it to our AGPLv3 projects literally no questions asked, we just have to replicate the copyright notice and license file. Meanwhile they can't take our copyleft code without also copylefting any improvements they distribute. And with AGPLv3, network use is distribution.