You might care about their judgement and their state of mind and whether they'd be prepared for the job. I think understanding what someone did in a long gap is important - some are just very easy to understand (person in 20's decides to quit job for a year to go backpacking) while others might be harder. If I was interviewing I wouldn't ever automatically discard a resume because of a long gap, though, so I suppose I'm just thinking of what others might be cautious about, and other people may simply adopt a heuristic to automatically discard to minimise variance.