why the letters in keyboards are arranged like this?

"I think you'll find this story enlightening. It includes the most thoroughly researched report I've seen on the origins of the QWERTY keyboard.
http://www.reason.com/news/show/29944.html

A watershed event in the received version of the QWERTY story is a typing contest held in Cincinnati on July 25, 1888. Frank McGurrin, a court stenographer from Salt Lake City who was purportedly the only person using touch typing at the time, won a decisive victory over Louis Taub. Taub used the hunt-and-peck method on a Caligraph, a machine with an alternative arrangement of keys. McGurrin's machine, as luck would have it, just happened to be a QWERTY machine.
According to popular history, the event established once and for all that the Remington typewriter, with its QWERTY keyboard, was technically superior. Wilfred Beeching's influential history of the keyboard mentions the Cincinnati contest and attaches great importance to it: ""Suddenly, to their horror, it dawned upon both the Remington company and the Caligraph company officials, torn between pride and despair, that whoever won was likely to put the other out of business!"" Beeching refers to the contest as having established the Remington machine ""once and for all."" Since no one else at that time had learned touch typing, owners of alternative keyboards found it impossible to counter the claim that Remington's QWERTY keyboard arrangement was the most efficient.

There's lots more relevant information at the site, I urge you to read it."

"The main reason we use a QWERTY layout today is tradition. It's far from the most efficient layout. Its letter placement is mostly arbitrary or based on flawed premises (e.g. the most commonly used letters should be scattered across the keyboard as widely as possible). Keys were even moved around so that early typewriter salesmen could show off their product by typing the word ""TYPEWRITER"" using only the top row of keys.

Edit: Check out this link for more information."