Because computers use binary systems i.e. 1 and 0 so it's called a binary system so therefore 1000 as a kilobyte is 2 to the power of 10 so that becomes 1024 so by definition a megabyte is 2 to the power of 20 and a Gigabyte 2 to the power of 30 etc. etc.

Injustice will be avenged.
Cofiwch Dryweryn.