Why is it impossible to make things foolproof?
Because they always come up with a better fool… I thought that was obvious!
Close!
Because fools are so ingenious? (Murphy’s Law # something or other)
Gold Star!
Because it is not desireable to do so. The advancement of science and technology requires an element of failure, so that improvements can be made. That failure may be inherent in the object or in its useage.
This is usually true of consumer goods. They have a built-in limited useful life to enforce replacement.
Most British Army equipment is ‘Soldier-proof’. Unfortunately, it isn’t sand-proof, water-proof, fire-proof or Arctic-proof. 
So, where does ambition take one to?
Apparently it’s impossible to make things nurseproof.
Somebody who was involved in hospital design told me this years ago, and gave a few examples. One was something to do with the oxygen, suction and maybe some other connections next to patients’ beds (I haven’t spent much time in hospitals). They all have entirely different fittings to prevent them being connected wrongly. I forget the details but some nurse managed a great feat of strength and forced a hose onto the wrong connection, with predictably unsatisfactory results for the patient.
Ambition raises one to ones level of incompetence! 
The Peter Principle!
Which applies with disappointing consistency in most organisations.
http://www.phill.co.uk/comedy/peter/index.html
… Principle (Peter and Hull 1969) which claims that"people are promoted to their levels of incompetence
Things don’t have to be foolproof; they only have to be useful in what they were designed for.
Bachstelze, Kaiten, Ohka, Katyusha . . .
Regards, Uyraell.