While true, it should be stated that the problems are a bit different. In quantum chemistry we are trying to solve the Schrödinger equation which we know, the problem lies in describing certain terms (many electron interactions) and then the approximations made to make the calculation time still reasonable. The bigger the system, the more compromises must be made, but there is an idea of what information we lose: upper bounds, lower bounds etc.

If you really had an infinitely powerful computer, in a way using complex theories such as quantum mechanics makes things simpler because you are dealing with the real fundamental interactions, where the true solution must come from, whereas in less heavy simplified models we try to take into account this behavior with simplified models which in the end lose information while still having to describe the same behavior.