why would that be? They certainly already need to put in a license page/dialog/display somewhere, so it makes so little difference adding an extra piece of text there.
This is certainly an issue with things like photos and Creative Commons. Anyone who uses CC-BY-SA photos in presentations knows very well that, as photos get reused and mixed in different presentations, it's very easy to end up dropping a credit somewhere. I know I try hard not to do so, but I'm sure I still miss things from time to time.