Why Korean buy expensive Bitcoin?

I am Korean and living in United States. When I buy Bitcoin or Ethereum , I use Coinbase. I can use Kraken, but they required too many things like scanning documents that I have to fill it up. Coinbase is easy to use, but I think they take too much commissions. When they show me the price at $89.45, but it is actually $89.93 when I try to buy it. I've heard if I use Gdax, I can set the price that I want to buy, but Coinbase on app or website, I can only buy at the market price; however, in Korea, you only can buy Bitcoin or Ethereum on the Korean exchanges. Top three Korean exchanges, Korbit, coinone, and Bithumb are well distributed, so prices are various every moment at each exchange. The high competitions among these three exchanges stabilizes the price. Unlike Coinbase, Korean exchanges have extremely fast deposit & withdrawals system. After you sell your Bitcoin, you can withdrawal Korean fiat money to your bank account within 30 mins, usually less than 10 mins. If you know the fact that it takes 7 days to deposit on Coinbase and 2-3days to withdraw from Coinbase, taking 10 - 30mins is enough to make you surprise. The thing is the price at each exchange can be firmed although it is a bit high due to easy deposit and withdrawals. If the Bitcoin price is more expensive at one exchange, you will sell it and buy it at other exchange that sell Bitcoin in cheaper price. High competition and easy deposit &withdrawals make higher price of any other concurrency such as Ethereum, Ether classic, Litecoin, and Ripple. These are major coins that three exchanges deal with, and all of them set up at higher price. If you buy bitcoin in US and sell it in Korea, it is arbitrage trading. The problem is Korean exchanges require your Korean phone number that must match with your name and Korean bank account. This barrier lets Korean exchanges separate from the price of other countries'.