Because the price of each bitcoin in those exchanges varies because of the volume that each one of them have at the moment.
If as an example, poloniex is having a lot of volume and suddenly btc drops to $7700 while bittrex is selling those bitcoins at $7850, you can make profit by this just by withdrawing those coins, depositing on bittrex, and selling for tether, and doing the same scenario once and once again.
It is not difficult, it is called arbitrage and it can be done in a lot of exchanges because there is a lot of difference in a lot of them. It is just another tip if you want to make an easy money, obviously that it is a little bit more difficult than trading.
So if i have huge amount of bitcoin like 1000
BTC can open site and set price like i want ??
no, because it is only easy in theory.
in practice you are going against the whole market. the trading volume of bitcoin in past 24 hours has been about 550,000
BTC, your 1000
BTC will be lost in that sea.
also you have to do it consistently which means every second, every day to do something to control the price specially if you are going against the market like dumping for example. a one time thing won't work.
that is why manipulating the market is becoming harder and harder as it becomes bigger.