I don't agree with a lot of Larry Wall's opinions, but during one interview, in response to a question about Perl certification, he once said:
My approach to language design has always been that people should learn just enough of the languages to get their jobs done.
I couldn't agree more with this. I don't know 85 ways to use unpack and haven't memorized perlvar simply because I don't need to know them that well to get my jobs done. This of course may change over time as I pick them up while doing other things, like reviewing someone else's code but as it stands now, there are other things I want to learn more.
Does the nature of programming and the fast-pace of computer-related fields contribute to this phenomenon? Does it in fact, induce it?
Well I don't think you'll ever run out of things to learn in this field. It obviously changes rapidly and in order to remain employable you need to constantly acquire new skills and improve existing ones. If at the end of every year you look back and say "wow, I can't believe how dumb I was" you're on the right track :).