It happens, and people have many reasons for leaving a bike standing. Be it family commitments, kids rocking up, illnesses of themselves or others they need to care for. Injuries after an accident can put someone off for a while, even just mentally after the physical injuries have healed. Sometimes, it's just minor breakdowns that they never get around to fixing, or financial difficulties prevent them from fixing, priorities etc.
Of course, when buying second-hand, a low mileage bike that's been sat for a long time can easily turn out to be more hassle than a higher mileage bike that's been ridden and maintained regularly...