why the letters in keyboards are arranged like this?
They were arranged in the QWERTY order so that keys wouldn't be easily broken. In the very old typewriter days, if you hit too many keys too close to each others too often, they would start interfering with each other. The QWERTY method was designed so that keys that are usually close to each other in words are not so on the keyboard. See http://en.wikipedia.org/wiki/QWERTY for more information.
Old typewriters have long arms for each key on the keyboard which have the "type" (i.e. the outline of the letter) at the very end of each arm.
When you press a key the arm swings out, and through the ribbon makes its imprint of the letter, and then swings back in again when you release the key.
Each arm is angled so that they hit the same point on the page, but if you press two keys that are in the same part of the keyboard in very quick succession these arms can (and do) collide.
The QWERTY layout was supposedly then designed so that most words would use letters from opposite sides of the keyboard, which reduces the risk of the arms colliding.
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.
What makes you believe alphabetic would be better?
When typewriters were being developed there were many layouts and many competitions to find the best layouts. QWERTY won - it was the fastest and jammed the typewriters less often.
http://en.wikipedia.org/wiki/QWERTY#History_and_purposes
Now we have electronic keys and the mechanical limits don't apply but nobody has demonstrated that alternaitive layouts would be better - not even Mr Dvorak has shown his claims to be true in real world tests (I'm sure I'll get lots of angry comments for saying that but it's true).
Also, QWERTY has a lot of inertia. Changing keyboards would annoy an awful lot of people and this is a big factor.