Because they have been in use for over 100 years and people just know not to touch the prongs.
I find it very hard to believe if that is just a personal subject thought, or something actually being considered?
For the last one, I hope not, because from a bigger perspective, that is extremely poor argumentation.
Most schuko sockets used to be also totally open (as in, one could put a pen, or metal object in it).
I forget when it was introduced, but since a few years a "child safety" little press cap inside the sockets in mandatory by standards.
I absolutely hate them (also remove them on my own bench), but I totally get why it became mandatory from a bigger picture.
Before people also knew very well that it was not safe to put a metal object inside those things and it also very rarely happened.
In this case it's a cost vs hazard analyses.
It's not if it very rarely happens, it's about the issues that are being created WHEN it happens.
For any exposed pins, there is no excuse for. 115Vac or not, that doesn't matter.
Some other wire or something else could get stuck in between (I have seen it), and create a spark or something that starts to smolder.
I really fail to see and understand with any kind of logic why flimsy hanging plugs are a feature, I am really sorry.
Mostly because the other end is meant for this.
What people did in AUS and NZ was sensible to this, just add a insulating sleeve to the pins.
Easy to implement, very cost effective, and will prevent any major issues.