Given enough computing power and resources --both human and hardware-- one could technically simulate anything. However, in the context of a reality with practical boundaries, that is simply impossible.

Imagine being tasked with simulating an entire Raspberri Pi board. You would probably need dozens of engineers and likely more than a year just to write and verify the simulation code. And, of course, a supercomputer to run it.

Hardware isn't about isolated components. It's an arrangement of components on a physical circuit board with interconnecting traces and physical characteristics that interact with the finished board. And, all of this, exists in the context of environmental requirements (temperature, humidity, vibration, RF susceptibility and emissions, thermal requirements, etc.).

A "simple" DDR memory design consists of memory chips, the transmission line (traces) connecting to the controller (FPGA, processor, etc.), the PCB stackup (at speed, this matters a lot), decoupling capacitors, the power distribution system feeding them, thermal management and more.

And that's just two chips talking to each other in a fairly structured way.

Pretty much any kind of non-trivial digital logic. For instance emulating a typical ARM microcontroller accurately, while technically within the realm of possibility, would require a huge amount of effort; every bus and every peripheral's functionality would have to be replicated, including hard-to-gauge details like their power consumption. Even if you have access to the HDL that was used to build the chip (which you usually don't), such a simulation will never be 100% accurate.

spicyjpeg basically described the sort of thing I was thinking of, but even for more boring things like opamps or transistors, it's a big hassle to track down or design a simulation model.

There's a semi-standardized file format for simulation models, but there are still differences between simulators. And every manufacturer's website is a different kind of poorly-organized, so it's hard to even find the models.

The other thing is things where high frequencies exist, like switching regulators. It's possible to simulate them, but there's all sorts of parasitics (imperfections from the circuit being made out of real stuff rather than lines on paper) that impact the circuit. I've only used KiCad, so I don't know if the commercial software here can model these parasitics, but they can have a big on performance.

Things like pspice and microcap can simulate a lot. But if theres high speed digital and switchers on the board, simulating it all could take tens of hours, if not more.