The long answer involves a lot of technical jargon, but the short answer is that the compilation process turns high level source code into something that the machine can read, and that process usually drops a lot of unneeded data and does some low-level optimization to make things more efficient during actual processing.

One can use a decompiler to take that machine code and attempt to turn it back into something human readable, but will usually be missing data on variable names, function calls, comments, etc. and include compiler-added optimizations which makes it nearly impossible to reconstruct the original code

It’s sort of the code equivalent of putting a sentence into Google translate and then immediately translating it back to the original. You often end up with differences in word choice that give you a good general idea of intent, but it’s impossible to know exactly which words were in the original sentence.