Because the block changed. We he finds one, its a whole new block by then. All his work on that last block will never be given credit.
That's just not the case. He has the exact same chances per hashrate of finding a share before the block changes as a high hashrate miner. Just with a higher variance.
If the block changes every 30 seconds on average, and you find a share every 30 seconds on average, and someone else finds a share every 5 seconds, how do you have the same chances?
You'll completely whiff on any given block just as often as you get one share.. meanwhile the other guy gets 6 in. If he has a little bad luck, he gets 5. You have a little bad luck? you get 0. But, it all evens out right? A little bad luck this time, a little good luck next time. Of course every time you have a little good luck, you don't get 2, you'll still get 1. He'll get 7
You each have a good block and a bad block. You have 1 share. He has 12.
1/12 is not the same ratio as the ratio as 1/6.