They did...but the one or two conferences you attend a year as a student isn't even close to what I'd call "networking". And frankly, that kind of interaction doesn't get you closer to a job, and it doesn't matter even a little bit for getting a professorship, unless you're giving a major talk (which, in all likelihood, you aren't).
The best networking I did in grad school was attending startup-related events. That got me my first paid gig, which led to greater street cred as a developer. Attending events held by the career center was a close second, because those led to interviews. Everything else was fluff.
Although PhD is a lot of work, its non-fixed schedule allows for some kind of freedom that you can use for networking. For example, PhD is a great time to be involved with open source projects where you can significantly contribute to, which can bring a lot of valuable opportunities if you're good at it and the project fills a marketable niche.
I know that my PhD completely changes the nature of jobs I can hope to get - not really because of what I learned directly at my PhD, but for everything else.
The best networking I did in grad school was attending startup-related events. That got me my first paid gig, which led to greater street cred as a developer. Attending events held by the career center was a close second, because those led to interviews. Everything else was fluff.