the reason so many leave before the 30 day mark is that's all the vacation and leave that they can get and still keep their day job. If they are not doing really well by then, as in having caught 100,000 calories of fish and game or more, they'd better just go on home, cause they have no chance of winning. This has chanced a bit, now that you can split a million $ with 2-3 other people. At least half of the contestants are picked for their weaknesses not their strengths.The producers get to send home some of the crew when half of the contestants are gone. This saves them quite a bit of money