That's fine for email, but services like Instagram thrive on literal word of mouth with people saying it over the phone or over TV or radio. Watch some TV and see how many adverts include the Facebook logo or a hashtag (implying Twitter, although a competitor could steal that market overnight). It's not just about clickability, it's also about having an identity easy enough for people to remember to check out. 'I haven't looked at Joe_Blow's Instagram in a while' is an easy thought to form, having to perform a search to find the random string of the actual account name, which is unrecognizable for most people if the search returns more than one result - that's a pain.
There's nothing wrong with vanity tags. Similar to 'verified' accounts.
And if you're referencing me from your phone, you could alias me as @knowsnothing and change my icon to a koala if you wanted to. That offends me less than me having to come up with @awesome_sauce314 to use some photo sharing service, which I'd need to share among friends, coworkers, family, strangers, etc.
I don't disagree, I just think that most service providers find the 'first come first served' model easier to implement. Having said that I'm struck by how this seems to matter less and less on Twitter, eg many celebrities have somewhat random-seeming handles.
