I have had this same thought and I think that this kind of low hanging fruit does still exist although most of the really big stuff has been taken. I've thought to myself that I could have invented x or discovered y if only it hadn't already been done but truth be told I'm not giving these people enough credit. They existed at a point where information, education and travel were not as ubiquitous as they are now and with more and more people learning more about all kinds of fields than ever before, we're just waiting on the right person to stumble across these falling apples and tell us what they saw.