Perhaps people once said the exact same thing about the Boer war? The war of 1870? The Napoleon wars? Or in my country the war of 1848-50 and 1864? or even Caesars conquest of Gaul? Time passes, and I think that if you asked similar questions about WWI there would be an even smaller number of people that knew anything at all.
Don't get me wrong, I think it is dreadful that such things are lost in time.
Personally I think it has something to with that I study history at a "professional" level (well nearly there, still studying) at university, that I find it appalling that large parts of the population is living in ignorance about the deeds of our forefathers.
However, it cannot be helped. I think it is natural. Humans in general tend to concentrate more about the things in life that matters right now and less about the history that made their current way of life possible. Many people find it odd that we value these things so much, and collect the relics of a bygone age and read kilometres of pages about the subject. You can lead a horse to water, but you can't make him drink it. We could put an emphasis on WWII in our educational systems (more so than today) but I don't think it would help much.
I think we especially find the collective memory-loss about WWII appalling, since it is the last great conflict (well hopefully the last) in western Europe, and because of Hitler - Nazism - Endlösung. However Hitler was certainly not the first, and he will sadly not be the last to try to "cleans" the lands of so called enemies.
But why should Hitler and the war he began take precedence? Because he is closest to our own time?
I have no final answer for that question. Personally I find all history equally important. Each "story" in their own way.
History is many things. Most people tend to think that history has to be about great men and even greater deeds. But that way the smaller parts in the course of human history are lost. And that I find, is equally saddening.
I have made my peace with the collective memory-loss.
As long as there is at least one person remembering the deeds of our forefathers they have not lived in vain.