The cities are just small enough that doing anything to get the high-level trade buildings requires 100% dedicating your city to that purpose. e.g. oil/refining, etc.
Sim City 4 leaned towards this as well. You certainly didn't have to do it, but it was a major boost to have a city dedicated to just trash and heavy industry and a city dedicated to making power and a city dedicated to high tech jobs and a city dedicated to living.
Cities XL is another game between Sim City 4 and SimCity where the land your city is build on will not have all of the resources your town needs, forcing you to juggle two or more towns. Some locations in that game are only good for resorts, some are only good for oil, etc.
