You keep seeing robins because there are roughly 320 million of them within a 6 million square mile distribution. Extrapolate that, bearing in mind I'm not considering a lot of factors (like prey distribution, climate preferences, migratory patterns), and you get about 53 robins per square mile.
tl;dr You keep seeing Robins because there are a lot of them.
tl;dr You keep seeing Robins because there are a lot of them.