Some people think that all they need is the time to do it and anything can be done.
I was one of those about 4 years ago. I thought that i could write an MMO on my own with only minor input from an artist friend of mine. I was very wrong. One man teams can have great ideas and can implement small wonders, but they cannot finish "grand projects". It's just not possible.
The reason you can't do it on your own is to do with the size of the project. A bit obvious you may say, but no. The size of the project has more to do with the different aspects that appear, not the workload. For example, as a single man team, you will often omit to do any of the following:
document your work
use version control
follow a schedule
look at old modules and refactor them
These are incredibly important things to do if you are working on a large scale project. If they are not done, you end up losing a lot of your time to dead ends and laziness.
Thats what happened to my MMO, I got as far as a vast world with a skinned character in it, and then couldn't bring myself to write the design for a network module, so i just coded it, and that was the end of that. You can't just write a network module.
This "one man team" is a problem even in the workplace, because some people think that they can do almost all the work, to the point where they don't trust anyone enough to be able to do what they are asking of themselves. This can destroy a multi-person project just as well as it destroys a single man project.
All projects should be considered to be multi-person projects, if not, you get sloppy, lazy, and you start to get protective of the code.