Look at it this way.

Xero mentioned musicians. Compare the type of song that can be played my a single artist compared to a band. All single artists have bands, sometimes just contracted for the one off. Today, multi track music can be put together by a single person but pro's making money get outside help for areas which isn't their specialty.

It's not impossible to go it alone, but imagine the difference if three of your bored friends were experts in Graphics, music, and story writing... and they just - "helped out from time to time" for maybe a small share of the profits if there are any.

And going bck to open source, what if other pro's could glance though the code and point out problems which you may or may not consider changing...

It all suddenly makes a sigle person project seem awfully hard.

but it's not impossible.