Because wireless keyboards are just better.
I like leaning back on my chair, holding the keyboard on my lap, resting my legs on the desk (my desk is L shaped, so my arm rests on my right, with the mouse). Having a keyboard that was wired to it would be a pain.
Plus, it's for a laptop. I don't like the laptop keyboard, or any laptop keyboard, because my hands are heavy and the buttons aren't elevated, so as soon as I start typing I start hitting three at a time. It's tiring for me to hold my hands upright like I was playing a piano for long periods of time (such as writing long papers or Farking).