Claire de Lune, it is really unfortunate that the general screening misses these and a lot of types of cancers. But general physicals and checkups and exams and basic tests as we grow older are all designed to catch the majority of major pathologies with the majority of the population, which means that a lot of these cases slip through. I imagine the "non-routine" workup is different and more expensive than the basic check, and insurance companies would go bankrupt trying to fund all the extra costs of complete screening for all of their customers.
Maybe this means we should be on a socialist universal healthcare program, although in that case, even a Federal program would be limited in available funds, and countries with socialized healthcare don't have significantly fewer cases of cancer so far as I know.
This really, really sucks - there's no real easy answer. It just happened, and there's nothing short of major advances in biotech and genetics research that will allow for things to change.... it's part of the human condition. My heart goes out to the family.