It's rare, that's all.
You have to look at quality of work, work life balance, the area, commute time, cost of living, salary, 401k match, benefits, chance of advancement, company culture, job safety, bonus amount, job security...etc. For a lot of people the choice is a no-brainer, but there are comparable and even worse jobs out there.
I apologise for the seemingly privledged attitude, but I'm assuming most on HN are Software Developers, Engineers, Mathematicians, and Scientists which generally have options and change jobs on occasion. Every single person who changed has done a pro/con comparison. Even if it was a no-brainier, the comparison would've taken place subconsciously.
Maybe not (although I don't think that's vanishingly rare).
But don't most people do something like this when deciding what they want to do next even if there aren't any active offers on the table at all?