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.

How does it help though when the user inevitably uses it on their lap and covers those vents?

I had the same thought as you before and one of my friends said “many people don’t call them ‘laptops’ anymore but instead call them notebooks”.

I found that name very fitting. Barely anyone uses their (gaming) laptop on their lap anymore. Usually on a desk in a thing that holds the laptop up to ventilate.

Note: I don’t know the exact name of it in English.