The truth is that these project view getting students to stick around and continue contributing to be more important than the actual projects they work on over the summer, and those retention rates are fairly low.
However, 20% retention rate is better than the 0% it would otherwise have been.
And for those who choose not to continue, if the project is healthy and the code useful then it will continue to be maintained by the project. Remember that this is something a student comes up with in a few months time. We aren't talking about massively complicated projects here. If someone else doesn't pick it up, then either the project is in trouble more generally, or else no one really viewed the contribution as useful anyway.