Because i) not all programs benefit tremendously from -O3 and ii) it adds to the compile time and binary size? It would be great to have some optimization level between -O2 and -O3 so that only portions that have a potential to be improved more than, say, 5% are compiled using -O3. In fact the existence of `#pragma GCC optimize` does suggest that this might be possible today with some heuristics... (Or use PGO, which will have the same effect. But PGO is still a novelty in 2023.)