Open-source software by nature is very vulnerable to stack-smashing attacks. It's weaknesses are known to be easily exploited.

Google and facebook support have the most of their software are proprietary especially the ones that make them money.

Most of the cutting-edge software in medicine, self driving cars, aviation, game engines, military, government projects, medical imaging, etc. are all proprietary.

You might find very few high-quality open-source software, the thing is they are well-funded, the teams are picked and not just anyone can contribute, they are worked on my high skilled professionals, unlike the vast majority of github projects that are simply under-funded, poorly-written, garbage.