What is cherry-picked about them?

So, I don't have any expertise in political assassination (either practical or historical), but I assume that for every would-be assassin who by skill, resources, or luck manages to even injure their target there are a hundred who never get that close... and that for a target as well-defended as a sitting American President, I assume the ratio is even larger.

If I'm right, then picking Oswald as an example is obvious cherrypicking. It's not as bad as, say, looking at lottery winners to argue that buying a lottery ticket is as legitimate a way to make money as getting a job, but it's an error of the same type. Pointing to how lottery winners are no more educated or qualified than I am doesn't really help that argument.

That said, I seem to have misunderstood your point. Sure, it seems likely that a significantly larger fraction of sufficiently dedicated assassins have significant political effects than of equally dedicated politicians... agreed.