My take is that it's the sheer population density that enables it. Large, dense European cities like Paris and Barcelona also have a rat problem. Rats travel w

My take is that it's the sheer population density that enables it. Large, dense European cities like Paris and Barcelona also have a rat problem. Rats travel within a 100 meter radius from their nest in search for food, so more humans = more waste = more food. Even if we keep everything tidy there's always this 1% of assholes who litter. At the same time the denser the city, the more difficult it is to conduct waste disposal services because even the nicest garbage container is going to smell, so you want it appropriately far from buildings and garbage trucks are large vehicles, which struggle in narrow alleys.