Because for two years it was my job to make sure that decompilers didn't work on code generated by my company... It's so easy to throw automatic analysis tools of the track. It's easy to throw humans, and we're way better at this game than automated tools, at least for the time being.

To give you an idea of how easy it is, if you want the decompiler to drop the ball on a for-loop, all you need to do is something simple like increment by two and then decrement by one at the start of the loop body... The tool might be looking for the use of an INC instruction, but because you increment by two, the compiler generates an ADD... and now your tool doesn't know how to recognise the loop.