The idea is that having “many eyes” on the code, the vulnerabilities should be found and solved more readily. BUT… of course this is true only for big projects which actually do have many contributors.
Viceversa, while skilled (or high budget) attackers can still find vulnerabilities in proprietary software (think of how people still crack consoles and games), good-intentioned people cannot contribute to weed out these vulnerabilities.
But most important of all, it’s trust. There is no way to be sure that a proprietary piece of software is actually as secure or respectful of privacy as it claims to be. Whatsapp claims to have end to end encryption, but you can’t tell if it’s truly secure or if it has a backdoor or some other spyware in it. For OSS instead you can always read the code yourself, or trust the many eyes that did.
Some months ago it came out that as soon as windows 11 is boot up for the first time, it starts contacting services like Amazon and steam (allowing for tracking) before even asking for license and privacy agreement. The same thing could not happen in Linux because the code is open.
Also, by using proprietary software from big corporations you give them power over your infrastructure and your work, and possibly on other people interacting with you. Which is definitely not poggers.