Because in the brain, the hardware and the software are the same thing. To understand what processes the brain carries out would be to understand intelligence.

Well you do not need to understand the low level of physics (quantum physics) to make very accurate predictions about ballistics with traditional, high level physics rules. You could as well understand the overall principles of the brain and have a high level model for it instead of trying to understand its smallest parts. This approach is also very valid, it all depends on what your expectations are.

But we don't understand the high-level 'physical rules' of the brain. While gravity could be explained by simple equations, the high-level operation of the brain has escaped us despite a lot of effort.

This goes back to the top-down vs bottom-up debate in studying the brain. What you are discussing is a top-down approach, where we understand what the brain does and then figure out how it works from there. A lot of people believe the 'correct' path is more similar to a bottom-up approach, where we understand the lowest levels of the brain and work up from there. This may be more feasible because we may actually have a chance at comprehending what a single neuron does. But amazingly our understanding of a single neuron's behavior is still limited. Some scientists believe we would need an entire computer to properly simulate what a neuron does (and others believe a standard computer can't physically do it).

There's a third group of very pragmatic people who believe the best approach will be a compromise between top-down and bottom-up - meaning we may not need to perfectly simulate a neuron nor completely emulate the brain's higher level function to make progress in understanding how the brain works.

Sorry, but gravity is not "explained" by any equation. It is merely observed and described. Currently there is no clear explanation as what is causing gravity to exist. So, we do not "understand" gravity better than the brain, in a sense. It is still a mystery.

I am not saying we should treat the brain as a black box and not try to understand its inner workings, but as another commenter mentioned, it is about finding the RIGHT level to understand its workings.

No. Gravity is a perfect example of a thing which is understood at large scales (via general relativity and Newtonian mechanics) but not at small scales. Which is exactly the parent comment's point about the brain.

Indeed, simulating the brain using a level of abstraction we think to be correct may be a reasonable way to try to understand which level is actually the relevant one for the brain's most important functions.

understanding what processes the brain carries out goes beyond being aware of its structure and localised functionalities.

i don't see why this high level understand would be a prerequisite to running a simulation of the brain.

by the way, hardware and software is always the same thing. my simulation of a laptop will include a pattern of high and low magnetic charges on its simulated hard disc, without my understanding the software which those patterns ultimately actualise.