Mining is competitive -- that means that everyone is trying to get a piece of the sliver of the pie that's being divided up.
The more miners, the more mouths to feed.
When the BTC/USD goes up, the pie gets bigger. ($/BTC)
When rigs are added, the pie TEMPORARILY gets bigger (BTC/hour created).
When difficulty resets, someone comes into the room and says, "the pie is HOW big? Hey, let's cut it down to size and put it back into a standard pie pan!" (6 blocks/hour).
Oh, but there's also that pesky "how much is the average Joe willing to pay for a Bitcoin" -- so the pie can only expand so much due to market valuation ($USD per BTC).
You see, earlier miners have their rigs paid off, so the $50, $100, or $150/day they're making is "free money" and can be spent on more hardware. That's why you see some guys snapping up 12 5830's when they become available for 10 minutes. Meanwhile, other miners are tempted to buy MORE hardware to keep their income up, only they'll never make any actual PROGRESS on paying off their rigs. Difficulty goes up, so income goes down. If you stop spending, you will get a smaller and smaller income.
It's not just "I have 1 rig, so I have one ticket. That guy has 10 rigs; he has 10 tickets". No, he has 10 tickets at this stage of the game, which means he has 10 rigs earning money for him. Even if you bought 9 rigs you'd be behind, since NONE of them would have paid for themselves yet. The guy with 10 rigs is still ahead of you.
Understand?
Yep, the "rich" get richer and "poor" get poorer.
It has always been thus, and thus will always be.