Yaustar basically answered everything as I would have, but I'll just expand on #3.
People want to work in big companies for a number of reasons. Such as:
- Larger companies means they are more stable and less likely to go bankrupt (I have yet to resign from any games industry job, they have all either run low on money or been taken over by someone else)
- A bigger team means you can work on bigger games. Current AAA console games take a LOT of man-years to complete, and who wants to work on a poor game?
- In a bigger team there's more room to specialize. You could be just "an AI programmer", rather than having to program the Ai, the tools, The physics and everything else.
- Big companies have big marketing budgets. You can make a great game that is everything you wanted it to be, but if you can't effectively tell people about it, you're not going to sell much.