They don’t.

I think the oldest thing I’ve played is mostly just NES stuff. Some of those will have been ported arcade titles or whatever, otherwise it’s plain ol’ SMB1 (1985, I think). I still play SMB3 ('88) quite often.

In canon (hue hue) nah, “God did it” which explains anything.

Vagal tone, is the technical answer. A nerve (vagus) runs through your body and, among other things, slows the heart. When straining to “void”, you cause that nerve to activate more strongly. It’s also related to the Valsalva maneuver.