The only possible reason I see people are buying btc is that they (truly) believe that the price will increase, and want to make more profit. I think that 99.99 % of the bitcoins exchanged are never withdrawn from the exchangers, and the people are only waiting to make some profits. What makes them believe that the price will rise? Because the amount of bitcoins is limited? Because lost coins can never be recovered? I have no clue.
Actually in most cases it also is the reason of purchase of bitcoin - investment with a possibility of growth. Well and of course with "buy-sell" cases.