Just as my anecdotal experience: I've shocked myself at least twice because my finger slipped over the contacts while I was trying to plug/unplug something in the dark. I have of course learned my lesson and am much more careful when plugging stuff in, but it's a relatively easy mistake to make if one is tired or not paying attention.
Yes, I got shocked because I was stupid and careless. But a connector that everybody uses every single day should be idiot-proof -- it should not be possible to kill myself because I plugged something in carelessly.
The US has the "excuse" of being the first domestic power distribution system -- other countries have better standards because they were able to learn from the design flaws in our plugs. And our split-phase 120V system is (as far as I know) a great idea that's way safer than direct 240V. But there really are a lot of things that the NEMA plugs leave to be desired. I second the recommendation by a sibling commenter to check out Technology Connection's videos on the subject -- I found them to be entertaining, nuanced, and highly informative.