My guess is that to some extent, YC benefits from having this around in a form people actually want to use - they are able to surface the projects they have fun

My guess is that to some extent, YC benefits from having this around in a form people actually want to use - they are able to surface the projects they have funded on it and that can help drive engagement to those projects. If they attempted to monetize the platform itself, it would likely drive away many of the users that would otherwise be interested in or be possible hires for the companies they fund.