Yeah, the convenience stores were one of the biggest surprises for me when touring Japan. I ate a lot of my lunches at 7-11 and Lawson, whereas I completely avoid 7-11 in the US.
While convenience store food in Japan is far superior to convenience store food pretty much anywhere else in the world (except maybe Thailand!), it's actually somewhat expensive and not as good as what you'd get in Japanese fast food chains. For example, a convenience store gyudon (beef bowl) costs more and is less tasty that Yoshinoya's standard Y300 gyudon.
I think it's mainly because you can't find a Yoshinoya in a town/road that isn't even on google maps but you can find 3 lawson's and a famlymart. (convenience≠cheap)
