The great sadness
So many things seem to be "a great sadness" to you that it's a wonder you don't live in a constant state of misery and anguish, Alan. Typically involving people not buying into your bizarre belief system.
is that so many people seem unable to even accept the possibility that they were brought into existence, not by accident, but for a purpose.
As a matter of fact absolutely everybody living or who ever lived was or is a complete accident. Three factors - facts, actually, and easily demonstrable - explain why this is so.
Firstly at least some people are conceived by complete accident - via random strangers having a one night stand, or a contraceptive failure, in other words by people who never sought to reproduce. Many, many places see birthrates peak in September/October. Now there's a simple (indeed, obvious) explanation for that, if you stop and give it a moment's thought.
Secondly, even if a child is deliberately sought and actively wanted, you don't get to choose what it'll be like due to the random (i.e. accidental) nature of genetics. Obviously it varies from one individual man to another but on average there are around 200-250 million sperm in every ejaculation - any one of which may fertilise an egg, or then again, it may not. (Even in the absence of contraception there's far more sex than there are pregnancies). Accident again. Randomness, if you prefer.
Stochastic, if you want the posh word.
Thirdly, even assuming that an egg is fertilised at all, there's no guarantee whatever that this will result in a pregnancy and a foetus carried to full term. This too is an entirely accidental matter. It's estimated that about two thirds of all fertilised eggs are spontaneously aborted through wholly random natural processes and end up flushed down the bog or some such - sheer accident, as I say. (This is why you hear people say, in response to anti-abortion cranks, that God for those who believe in such a thing is the greatest abortionist of them all).
So you see Alan, everybody is an accident somewhere down the causal chain of explanation. That doesn't bother me nor should it you - simply stop trying to find purpose and intentionality where there is none.
A purpose totally beyond our human understanding
Well, by definition it clearly can't be totally beyond human understanding or you wouldn't even know any such purpose exists, and we wouldn't have people like you claiming that there is such a thing.
Unless of course you're just pulling all this sort of stuff
ex recto.