It takes an immense amount of time to make something good and polished, which results in one not being able to do anything else (if you want to get the project done in a timely manner).
I know many people who have wanted to start their own 'indie' development studio developing games and apps for smartphones. It becomes a full-time job so that you cannot have an actual full-time job. Then, once it gets released, you barely get a return on the time you've spent on it. It's generally not a good plan.
That's why something like Kickstarter is such a good idea so that you've got the initial capital to spend on the project.