It's called arbitrage trading.  Here's why all the prices aren't the same:

-There's a "spread" on most exchanges, which is the difference between the buy and sell prices.

-Some exchanges have high fees (e.g. Coinbase)

-Some exchanges are dysfunctional (Bitfinex, BTC-E, Chinese exchanges), and some exchanges are also extremely shady/annoying when you move large amounts (BTC-E, Poloniex and more).

Overall doing arbitrage with several exchanges can get very hard.  There are some minor opportunities with the extreme price fluctuations that go on, but they're gone very fast.  I don't recommend you get involved unless you know what you're doing or you can trade with foreign exchanges (like South Korea).