Every piece of code is a potential vulnerability, really. Not just dependencies.
But code that you don’t own, that’s outside your control, is particularly vulnerable.
One of the big myths of using frameworks and libraries and cloud services is that you no longer have the “own” that piece of the code. You’re benefiting from someone else having already solved it.
We deal with this a lot at my job and I think it's important to take note of. We thankfully have dependency checkers to catch known vulnerabilities in the packages we reference so issues are hopefully caught and identified sooner rather than later. But the fact remains that we can be at the mercy of the frameworks and libraries to fix them.
Third party developers could abandon their libraries or they only fix it in a version that has breaking changes compared with the version you're using. Either way, it means that you're now in a bind with your website or app.
This is not to say don't use third party libraries or frameworks. Most developers are fantastic and are legitimately doing their best to write good software. But it should cause you to do at least two things. First, be mindful of what dependencies you use. Second, do what you can to make sure you, and/or your company support the open source developers who make the tools you use.