perhaps it's because you're insulting the very people you're trying to convert to your cause. work on your rhetoric and watch your success rate skyrocket
pretty sure the meme is about how the implementation looks ugly but using the implementation looks good because all the code is abstracted away. if it was like you said then why would they compare the code to the main.cpp
The reason is that header files are pretty much copy/pasted into your c files when you include them. so the code in them keeps getting recompiled for every c file, which drastically increases overall build times. If you only declare in the header and have one c file implementing the functions you compile them only once.
perhaps it's because you're insulting the very people you're trying to convert to your cause. work on your rhetoric and watch your success rate skyrocket