I don’t think the real problem is that the vulnerabilities exist. It’s a question of how many people are looking for those vulnerabilities and what those people’s intentions are. With big open source projects, as someone else already pointed out, the number of good actors far exceeds the malicious ones, so when a vulnerability is identified it’s more likely to be by someone who just wants to patch it, not exploit it for gain. In a closed source project, fewer good actors are looking - only the people allowed to work on the code - but the bad actors are probably pretty much the same. Of course, popularity of the program and what it’s actually doing matter, too, in terms of how interested bad actors are going to be.
I love the idea of open source software for exactly this reason. I see it as a reminder that most people are good.