We kinda... do? Draw the line there, I mean. Reverse engineering, as I've always heard the term used, is taking the final artifact and working backwards to infer the original design, and ideally some of the reasons for the decisions made. If you take a shipped binary, disassemble/decompile it, figure out what the variables mean and how it all works, that's reverse engineering. It's the equivalent of taking a mechanism, pulling it apart, and figuring out the cause and effect of how it works, to the extent that you can duplicate it and even modify the functionality.

Starting from high level source code is like starting from engineering drawings or the CAD model. You've already been handed most or all of the info that reverse engineering is attempting to recover.