I prefer GitLab but I understand why GitHub is the default. Just different products and use cases.
That's kind of what it was designed for, though. GitLab.com wasn't a popular choice for open projects compared to GitHub until the "big migration" several years ago. Before that, GitLab was very popular for self-hosted internal instances (their customer reel demonstrated that). Even before modifying their OSS org policy for self-hosting, many groups ran their own GitLab CE instance. You couldn't (and still can't) do this with GitHub*. It also had a longstanding unlimited private repos compared to GitHub's free tier (formerly) that enticed developers.
GitLab's UI/UX was made for business workflows and processes (hence it being an "all-in-one DevOps platform." GitHub leans more into the community graph and semi-social media style for orgs/communities (the Discussions section a case example). It has come a long way for the business side, though.
* Yes, GitHub Enterprise Server exists: good luck getting it without paying for it (unless things have changed).
EDIT— here's a newer epic trying to gather some of that together into a cohesive effort: https://gitlab.com/groups/gitlab-org/-/epics/11247
Sometimes it has far too much discoverability.* It seems some searches are global. For instance, when I go to `My Merge Requests` and try and change the author, it shows me every single account on GitLab.
* One could argue this wraps around and becomes undiscoverable. Which I would agree with.