Just that they aren't used much -- they certainly aren't extinct, but
compared to my antique radios they aren't used much at all.
My late stepfather had a collection of Tobacco tins with his collected
resistors, quite a few as he worked for Mullard during the war and up to 1980,
and when I was at school (1950'd - 1960's) I can remember the hexagonal bodied
resistors with their colour code dots of paint that he had in them. They were
solid carbon then, we used to file nicks in them to change the value...
I megohms was a huge value, used for grid leak resistors IIRC, and he had a few
in there! 470K was the regular high value resistor that everyone had, but the
megohm values were indeed rare.
Peter