Because "people" aren't even aware that this stuff can happen. I actually think it's far more likely that in this situation the other person did a search, but most people wouldn't even consider that a simple search could result in you ending up on a "People You May Know" list.

TL;DR: people don't know because they aren't savvy, and that isn't their fault. Those of us who do know should do a better job, but no, I have no ideas how either.