They work fine with people, can parse small talk and bring a positive attitude, etc. But they were out like a shot when the workday was over, and they would never dream of attending a company-sponsored teambuilding event. If they were hired for a 100% remote job, they would expect to never see a coworker in person, and consider it a perk.
Some people just don't want to mix their personal and professional lives. What's wrong with that?