One can always change street addresses. But it's a royal pain to create new usernames.

I think the point is that we expect our phone numbers to identify us in the real world but don't expect them to point to our handles online. If I use the same username on Snapchat that I do for something embarrassing like, say, my tumblr, someone who knows me IRL can look me up and find my more private online presence. This is just as real an issue as the reverse scenario in which Internet stalkers find out my phone number or something.

Now generally it's advisable to use a unique handle (or your real name) for a service so closely tied to a piece of real life identification like your phone number, but I don't think a lot of people do it.