As an open source author and maintainer, I think you're looking at this from the wrong way around. Good contributions are not the norm, they're the exception, and from the offset it's often not so straightforward to figure out which one you're dealing with. It isn't constructive to think of a contribution as a gift, if anything, it's a Greek gift [0]: contributing to an open source project creates more work for the maintainers, both in reviewing the contribution and then later on maintaining it. If the contribution is good, it offsets those costs, but oftentimes it is not the case. Don't get me wrong, there are countless brilliant people out there who are doing great work more or less for free in the way of contributing. I'm also very grateful for good contributions. Those contributions are the exception though, not the norm.
[0] https://en.wikipedia.org/wiki/Greek_gift_sacrifice