why do people hack?

Because they're bad, and they can't face feeling like they're helplessly bad...
Maybe even they're hacking because someone else hacked and made them loose ranks or hurt their K/D or Win/Loss ratio in a way that's irreparable... And they feel that either the only way to get back up is by hacking, or by hacking just to get revenge against people who hacked against them...
I find this especially true in competitive games like Fortnite, PUBG, Counter-Strike: Global Offensive and Overwatch..

Especially when you have really sub-par AntiCheat systems like VAC....
There's so many holes that would be so easy to fix... But for some reason Valve is not putting a ton of effort into VAC, or at least it seems...