That's... the whole point of quantum computing, and why Feynman [co-]invented the field. Classical computer simulations of quantum systems cannot scale. Quantum computer simulations of quantum systems can. That means that if you want to simulate a quantum system, you need a quantum computer. Or you can simply observe the quantum system and attempt to infer details of how it works from the observations. Or you could choose not to understand some quantum system, but that's not what scientists and engineers would choose.

My interpretation is slightly different. Classical computers are slow because they need to simulate the collapse of a wave-function using probabilities with something like Monte Carlo. Quantum computers don't simulate anything, you build a replica wave function and then measure it classically, forcing the collapse to happen in nature. But this has an obvious hard limit where you can't simulate more qbits than your quantum computer has.

Probably two things. On one hand, our theoretical understanding of quantum effects may not have come far enough to write a traditional simulation program. Or doing so would take years or even millions of years, or both.

Quantum computers can simulate things that are not possible with traditional computers. These problems are in the complexity class BQP. I want to point out though, that this is very likely a result of our limited understanding, and not something that is theoretically impossible.