I don't think it's just inertia, at least not in my case. I know that my example it's just anecdotical, but I'll write it down anyway.
I'm in my early '30s, I've been programming in Python for 7 years now (and during the last 6 years I even got paid for the privilege), along with other languages (PHP, JS etc.)
The thing is there's only so much that you can learn about the world around you by focusing on only learning programming- or tech-related subjects. Of course that, for example, learning Erlang or Caml are extremely interesting things to do, intellectually speaking, but when I realized that by learning to speak Arabic of Farsi I could potentially be able to interact with and to better understand the culture of tens if not hundreds of millions of people then my interests slightly changed.
Of course that it wouldn't help me one bit if I were to tell my potential future interviewer "hey, I learned to speak Farsi so now I can really read Hafiz's poetry the way it's supposed to be read", but as long as that experience enhances my understanding of the world around me it's all for the best.
Re-reading what I wrote I realize maybe it doesn't make much sense, anyway, what I wanted to say it's that most of us, programmers, are (really) smart people, it comes a point in one's life when you realize that there are other things in life worth spending your time on.