Adding one extra dimension to a situation often adds a lot more complexity than you would think. For instance, chess is a well-known game in two dimensions between two players, but try to add a third player or a third dimension to the board and everything gets way more complicated. There are so many ways to do this that no one option has ever emerged as standard. Or good old Spirograph. Try even imagining shape drawing like that in three dimensions and it quickly becomes clear that it's nothing like on paper, assuming you can even figure out how it should behave. The point is that simplified problems can help you get a feel for the more complex variants, but there is always an extra element that means the simplification is inadequate.
Mokalus of Borg
PS - Things are often more than the sum of their parts.
PPS - Especially when you're modelling complex systems.