> By this logic, programmers should be able to accurately estimate the schedule for personal projects or small scale open source projects where no one can "reprimand" them.

In my experience many good and experienced programmers are able to do that. The reason why you get different observations is that in many cases, if you do such a project, you do it because the journey is the destination, i.e. you do such projects to try out new interesting technologies, i.e. the central gola is not finishing the project, but to get exposure to programming topics that you consider interesting. Finishing such small scale open-source projects to a given maturity is just rather a desired side effect and not a goal.