You don't work around dangerous things assuming you'll never make a mistake, you work around dangerous things assuming you'll never make three mistakes at the same time.

You are not immune to making one (or more) mistakes, no matter how careful you think you are.

Well, maybe it's because you may die if you accidentally touched touched the prongs? The purpose of female plugs is among other reasons to prevent accidentally touching them.

In addition to the exposed prongs, it also means you are passing current into a circuit of unknown capacity without using a safety breaker. You may also be back feeding into your neighborhood power grid and can kill people in the street/other houses that were not expecting the lines to be energized.

The problem really is the super exposed hot prong you now have once you plug one end in

50-50 chance whether those two outlets are on the same phase or opposite phases; if it's the latter, congrats, that's a 240V short.

Besides, if there's an outlet at the far end of your string of lights, you don't need this, you just plug it in there

Two things: 1: there's a high chance you do cross live and neutral, or even live and live on different phases. 2: using it to plug in a generator to power your house can kill electrical workers who are trying to restore a power outage. (If you fail to open your circuit breaker.)

Why would you "need" one of these if you could plug in the other end into a second outlet? The point is that idiots don't plan out their christmas light layout and end up with the wrong end at the outlet. They decide that they would rather drive to the hardware store and buy/build a suicide cable rather then just taking the lights down and rehanging them or running an extension cord to where the male end of the lights are.