Because 'right click the ISO and select mount' is hard to explain.
I do agree with the rest of your post, and understand that it is often easier to explain a command line process than where to click, just not this instance.
I'm all for the command line, and think everyone should know what it is, how to get at it, and find out the right commands if needed. However, for most it's not always worth it. If one was to learn a specific command to do this or that, and didn't require it for 2-3 years or more, they aren't going to remember it. It's more reasonable to GUI it.
If my car is scheduled for a timing belt change, I'm not going to teach myself how to do it, plus buy all the required tools so I can so it again in 10 years. Most don't even service their car out of the same fear/unknown/laziness as one not using the command line.
Apologies for the clumped reply, the android browser really doesn't like /.