Why doesn't the government treat illegal immigration as the crime it is?

How can "sanctuary cities" be legal? Isn't that a city nullifying federal law? Why don't local, state and federal governments do more to round up and deport illegals? It seems like both conservatives and liberals have their own reasons for allowing illegals to invade and remain in the country. Shouldn't they be held accountable for aiding and abetting criminals? Shouldn't this issue be given a lot more attention than it gets? Lastly, why do some people believe it is okay for illegals to remain?