Some Facts:

• An eruv cannot be made in a reshus harabim.
• Many eruvin are made in cities nowadays.
• Cities are much bigger now than they were thousands of years ago.

Yet it seems that in those days, people refrained from carrying in cities. This can be seen from the many enactments made to prevent people from carrying, which implies that many people lived in a reshus harabim d'oraysa. (Also, people could only carry in Yerushalayim because it had walls with gates that were closed, but a simple "tzuras hapesach" wouldn't do the trick.) So how can we make eruvin everywhere nowadays?