One reason, it is like playing the lottery. You know that your chance of winning is almost nonexistent. Yet people buy those tickets anyway. People know that they won't hit it big mining, but there is still that one chance. BTC exchange rate could shoot through the roof. The pool you are in might get extremely lucky. etc...
Ppl buy lottery tickers with 1$ with a chance to win 1.000.000$. They don't buy tickets with 5000$ with a chance to win 4000$.
Thats why the buy them "anyway"...
The BTC exchange rate is not an argument as well because you can buy bitcoins directly instead of miners.