I don't have any special insight into the internals of both games, so I can't say for sure. I certainly think you could make a tower management game that has a simulation engine that is just as complicated as SimCity 4 or 5. On the other hand, I think that in principle a game attempting to simulate, to a degree, an entire city is bound to offer more complexity than a game attempting to simulate the goings-on in a single building.

I can make an argument from observation: Sim Tower is a 68030/386-era game, and from what I remember, my 68040 ran it very well. SimCity 4, OTOH, can still bring (a single core of) a modern CPU to its knees. Of course it's possible that SimTower was incredibly efficient and SC4 incredibly inefficient, but it seems much more likely that SimCity 4 is in fact vastly more complicated.