It’s by necessity. The hardware in a gaming laptop gets a lot hotter, so you need wider openings that are closer to the heat generating parts. If you were to do this on the sides, you’d need to build the case bigger and put a much stronger heat transfer system to account for the smaller and further opening.

Very few folks use a gaming laptop on their lap without some kind of stand or hard table designed for it.

Battery life is too short and you need a place for a mouse.