because you want to have a "sick road bike" getting rained on, left locked anywhere in a city, ridden in shitty conditions, ect...
and pay for the upkeep?
I ride a fixie because I like the added challenge, but anyone can ride a singlespeed coaster with a front brake, and theres almost nothing to worry about on them. Unless you're racing in the tour de franzia, live in a mountainous area, or regularly go on rides longer than 10 miles there really isn't any reason to get a "sick road bike"