Yeah, they usually only put them up in neighborhoods with a large portion of jewish people since it has to be checked for contiguity before every sabbath. That alone limits how large a portion of the city you could enclose.

Yeah, they don’t usually encompass the whole city, just the Jewish communities. Eruvs aren’t really necessary if you aren’t following Halacha (Jewish custom/religious law). Plus it’s a big deal if it’s broken, and it’s less likely to be broken if it’s smaller and easier to maintain.