Because work places in one place where you must interact with your coworkers to do your job. The bar is a lot lower for what constitutes harassment

There is a certain level of professional conduct that people expect in a professional setting that’s not there in school.

Also, I would be very wary of approaching someone at the gym who was working out. I use to teach at gyms part time when I was single. Women would have to really start the conversation with me before I would make any move.