a) Because I stare at a computer screen ~10 hours a day at work. When I am reading in the evening I prefer to have a book, which I can underline and scribble in.
b) Every programmer over time develops tiny helpful apps that make life bearable, having to rewrite all those apps / utilities in a fresh new language is a burden that I would like to avoid.
c) Admittedly I am a bit more aggrieved than most people because my languages of choice are lisp, scheme and python.
I prefer to read from books but the information in most programming books is out of date very fast. For this reason, I prefer learning languages from the web.
For the same reason, most information you find on blogs is also out of date.
Also, learning a new language or library that's different enough from what you know that it takes more than a few hours to be comfortable (generally) makes you a better programmer.
Sorry, I do realize you were mostly trying to pile onto the IPhone cost tally, but I'm always surprised to see other programmers imply that learning new things is inherently hard and undesirable.