Many many times more fans only watch football on tv, and like anything entertaining enjoy watching someone good and thus end up supporting whoever was entertaining to watch when they first got into football.
Why should football be a local thing, think about it, should a disabled kid who can't get to football games not be allowed to find football entertaining on tv, can he not watch football on tv and come to enjoy watching a particular team? What about Utd's Asian fan base, should they be banned from watching higher quality football with the worlds best players because they were born somewhere in Asia?
The idea that football is anything but entertainment and that somehow it's strange to enjoy watching better rather than worse teams is nothing short of ridiculous.
For people who enjoy watching their local team, great, for people who only ever watch on tv and pick a successful team(why, dear god would you watch an awful team say like Stoke and CHOOSE to support them?), great, it's something to watch, nothing more, nothing less. Why does someone have to choose to watch football or pick a team the way you did?
Would you choose to in the future only watch terrible films made by a specific director because once you watched a truly terrible film made by that guy? No, why would football be any different?