Why are there over 600 open pull-requests in Scikit-learn github repo?
mean, reviewing a pull-request is not that big of a deal compared to writing commits and also this normally has higher priority than writing code, so why I see new commits in the project, but PRs are still not reviewed?


I assume that it's not that simple if they want to maintain the stability and the homogeneity of their code base. If they were just quickly accepting PRs the whole software would become an unmanageable mess in no time, causing problems for a large community of users. It's almost unavoidable for any significant software project to become harder and slower to maintain and develop when it reaches a certain size, especially if many users depend on it.

Also it's maintained by a community of volunteers so there might not be that many contributors who are both knowledgeable enough about the code and have enough free time to study all the PRs coming regularly.