They aren’t doing it out of the goodness of their heart. It’s a pain in the butt to maintain out of tree changes. Getting code upstreamed means less maintenance work.
In my experience the types of developers building this stuff are personally fans of open source and companies are willing to let them submit upstream due to PR wins and limited downside.
Companies, who make up a sizable portion of OSS commits, do it because their interests align. Why is this different, and suspicious or malicious?