Cynical answer: because having users writeup duplicate issues and then having maintainers close them is more engagement than warding off unnecessary toil. Gotta keep those metrics going up and to the right.
GitHub doesn’t have ads and makes its money off of enterprise subscriptions (and Copilot), so I don’t think “engagement” is a very important metric for them.
Wait if it's not their core product, what is? GitHub is, at its core, a file/history browser + issue management system + merge request system built around Git. There's not that much to it other than issue management.
It's core product is git hosting, you use it to host your git repositories. You use features such as Pull Requests to power how you merge within your git repositories. If the issue system isn't working it's not a big deal, but if we can't use git it's a massive deal. It's all in the name GIThub
Most companies don't use GitHub's issue management system they use issue management tools such as JIRA, Trello, etc. Issue management, project management, CI/Actions, wiki, discussions, etc are all nice to haves and are probably more aimed at the open source projects that are used as a marketing tool.
Most open source projects (you know, the thing GitHub claims to exist for) do pretty much exclusively use GitHub issues for issue tracking though. GitHub makes it pretty difficult to be on GitHub and not accept GitHub issues.