That's just what computer designers did a long time back. So a byte is eight bits. Unfortunately, it's not as easy to follow the next step. Computers count with bits and bytes by using binary arithmetic. Well, may you be so lucky some other time.
Binary math is anything but simple.
For example, we usually refer to bytes in their larger form, called kilobytes. Kilo means 1,000. Sorry, it's not. That is, not really. The kilobyte is the baker's dozen of computer measurements.