I think it follows from two things:
1. We use open source software for everything. This is also true of our dependencies, so we get hundreds or thousands of transitive dependencies. Many of which are presumably written by dogs, because (i) no one can tell if you're a dog on the internet, and (ii) OSS maintainers are so overworked they ask their dogs for help.
2. Languages and libraries are full of footguns, software is full of bugs and therefore vulnerabilities, and no one cares enough to go through the enormous effort to fix things. And this is true through the whole stack. So the only practical way to stay secure-ish is to reactively patch software as vulnerabilities are publicly discovered. And also defence in depth. (I would distinguish between the publicly known time that a vulnerability is discovered, and the first time it was discovered. You hope the two are the same but for many vulnerabilities, if a clever adversary found them first we'd never know.)
With these two things together, you have a ton of questionable dependencies, and you need to update them all the time for security reasons.