Assuming right handed with right handed scissors, looking down at what you cut:
You put a couple fingers through the bottom loop. Most people don't pull those straight upward, but pull a bit toward their hand. This pushes the blade to the left as it closes. Likewise with the thumb you push away a bit, which pushes the blade to the right. Thus, both blades are pushed against each other.
Sure, you could balance the scissors perfectly and only go straight up/down. But this won't work very well when cutting something difficult since there are no forces balancing each other out. It also won't work with looser scissors (every pair in elementary school) ad the blades will remain apart from each other.
You put a couple fingers through the bottom loop. Most people don't pull those straight upward, but pull a bit toward their hand. This pushes the blade to the left as it closes. Likewise with the thumb you push away a bit, which pushes the blade to the right. Thus, both blades are pushed against each other.
Sure, you could balance the scissors perfectly and only go straight up/down. But this won't work very well when cutting something difficult since there are no forces balancing each other out. It also won't work with looser scissors (every pair in elementary school) ad the blades will remain apart from each other.