IMO, it's because understanding someone else's code and introducing your own changes is something that requires a considerable amount of experience and knowledg

IMO, it's because understanding someone else's code and introducing your own changes is something that requires a considerable amount of experience and knowledge which many new developers have not acquired yet. Instead of trying to take the time to understand how to integrate the changes they're trying to make with an existing project, they create something new practically centered around this new feature and a lot of stuff gets lost along the way. At the end of the day, it's fine though. It'll either satisfy the needs of people and become a new standard or it will be satisfy edge cases and become the new thing that needs to be deprecated because no-one contributes to the project and the original maintainer has lost interest.