Two reasons: 1) The hashing difficulty automatically adjusts to match the network throughput, so that new blocks are found once every 10 minutes. New blocks i

Two reasons: 1) The hashing difficulty automatically adjusts to match the network throughput, so that new blocks are found once every 10 minutes. New blocks introduce new Bitcoins, so this controls the total supply. 2) New blocks extend the last agreed-upon block. If an attacker forged a block, they would have to do so faster than the rest of the network combined to control the entire block chain (clients agree the longest block chain is legitimate). While you could do this without the same computational expense, it would rely much more on Internet connectivity. An older AMD GPU can calculate ~250 million hashes per second. With no difficulty, a transaction could be verified on the network as fast as miners could broadcast their agreement. This would mean attackers could more easily control the block chain with DDoS and other network-related tricks. Even now, the largest mining pools are sometimes targeted to slow down transaction processing.