Because your "average" user will never look anywhere but in the store.
If you are reading the Register you are not an average mobile phone user. Neither am I but knowledgable users are a small minority...."
It's also why the constant stream of "I switched to Linux xx years ago and haven't looked back" comments become tedious. Yes, Linux is awesome. Yes, it's desktop variants are almost as simple and straighforward to use as any version of Windows - in some cases, arguably easier.
But the point all too often missed here is that none of us are atypical users of either phones or computers.
The same people that buy an average PC from PC World, Dell's webiste etc, don't give a jot about the OS other than, perhaps, wanting something similar or the same as they have at work.
Remember, we are still talking about people who are lost if you move an icon on their desktop! You are talking about a large percentage of a workforce now, who have only used Office with a ribbon - it's been around 12 years!
Also - remember how we are always saying we shouldn't teach non-technical users bad habits? Telling them to install random/semi-random apps from a website is a slippery slope and it's only a matter of time before they're being yelled at "why did you enable untrusted sources, you fool!?"