It takes about eighty people working more than full-time for two years to make a big game. (Some take more - Assassin Creed 2 was around 130, I think some take less). These people must be real experts in what they do, and you need many different skills: programmers, artists, writers, actors, sound designers, level designers, producers, QA.
Suppose you want to make a world-class game that will compete with the best graphics, graphics, art, sound, design, the whole offer. You need world-class people who do the job: for example, animators who would otherwise work full time at Pixar or Weta. To get someone to work for you full time, not Pixar, you have to pay them a lot.
The game is not the thing where you can take what will be 40 hours of work for one person, and distribute it in an hour of work to forty people. This requires a lot of difficult, untimely work. This is not just programming the graphics engine - it tests the same broken thing over and over again, fixing an error that only appears on a Windows Vista computer that has a special ATI card installed, drawing bumps for fifty slightly different types of boxes. Lovers of volunteers tend to "scratch their itch", do what they are interested in, and leave it to someone else to polish.
It takes a lot of capital to play. Each developer requires a high-end workstation, sometimes two. Large screens. Fancy pills for artists. Maya (no open source tools, even remotely comparable). Do you make a console game? Development kits are 10 thousand. Dollars. USA apiece. Performing motion capture? 500 dollars per hour for studio rental. Hiring voice actors? The SAG scale starts at $ 800 per day. Some of the guys from the forums will not be able to fulfill the role of a professional result. Plus electricity for all this, a building to put it.
This is expensive, and it requires a lot of specialized knowledge, working over time, even when they are tired and stressed, and actually disagree with Creative Vision, but still need to finish the job. It will be difficult for you to convince really talented people to do this for free.