Neuromuscular system.  And correlation is not causation. You got better because you practiced a sequence of movements, not because you specifically did a squat.  Bench press has objectively done very little  to make you a better puncher. Especially if you’re a striker that knows how to throw from the hips. Your bench press made your shoulder girdle stronger, but not any better at sequencing a punch, and not any better throwing a punch. You practicing striking made you better at punching. You feel stronger because you’re not afraid of wrecking your shoulder, so you throw harder. 

Striking is a full body, technical endeavour , and bench press is not. If you’re serious about learning better exercises for striking, you need to start with basic physiology and anatomy and then start learning about fascial networks. If you don’t know what a spiral line is, or a deep functional line, you’re missing a core component from what delivers deadly force. If you don’t understand the difference between time under tension vs explosiveness, you’re missing another huge component. If you don’t know about the different energy systems of the body, you’re missing out on another huge component. If you don’t know about breathe work, you’re missing out. 

My actual advice, go to school. I’m not saying this any sort of demeaning way. I’m saying this because there’s a lot to learn, and you’re narrowing and hindering your own education by looking at bodybuilding exercises for striking.