I researched into it more and turns out it's called "Key rollover", and it comes from your keyboard. Basically, the better wired your keyboard, the more keys you can hold down at once. 2KRO is the worst key rollover, NKRO is the best.
I was making a game for a game jam when I realized this. I needed to press multiple keys at once, but I couldn't. Turns out, this is why.
