A lot of people don't. Hell, my last job change occurred because I was contacted by a recruiter from what was literally my dream job for the past decade (you don't ignore an email like that). I was content in my previous job and not actively looking, and had actually been promoted to lead developer not a year prior.
The only recruitment calls I ever get are from body-shop class contracting firms that never manage to even come close to my current compensation for scut work.
Optimize your LinkedIn as you would for SEO purposes and you should get plenty of contacts. You have to also know a bit about LinkedIn's algorithm - for example, a profile without a photo will be considered more incomplete and thus score lower than one with a photo (doesn't matter what the photo is obviously).
If I had to guess, it's because I interviewed with them when I was still in college and didn't get the job, but remained on their radar for later follow-up. This ended up being seven years later though, so who knows.
Another guess is that I had a recently listed promotion to Lead Developer on my LinkedIn account. Anecdotally I've heard similar experience from other people who started getting more high quality contacts once their job title on LinkedIn reflected a more senior role. I know we all hate LinkedIn but it matters because people on the hire side use it extensively, so spend some time entering details in there so that it's as impressive as your resume.
Do you not look around unless you are miserable?