I would say: when the existing offerings completely prevent you from doing what you want to do, or require ugly workarounds that are not consistent with your goals, or when future changes in those dependencies might compel you to do extra work just to keep your own system running.

One more reason to start from scratch: to get the functionality you want from an existing offering, you would also have to include a lot of other stuff you don't need, resulting in unnecessary complexity and resource consumption.