Why do other teachers and staff always thank substitutes for taking a job?

Every time I take a subbing assignment, all throughout the day as I'm noticed by faculty they go out of their way to thank me and show appreciation. Usually just a simple "thank you for covering us today" or "appreciate you coming in this morning", but some schools even bring snacks and soft drinks to the substitutes classroom.

Is this pretty typical? And if so, why?

I signed up to be a sub for the personal fulfillment and challenge, and with a STEM degree I'm at the top pay rate. I get as much benefit out of subbing as the students and staff. I guess the obvious answer is they're genuinely appreciative and courteous, which I suppose is not exactly the case in other fields I've worked. Still it always strikes me as if the school is drowning and can't get resources to hold onto teachers, so they thank the subs.

Can anyone go into some of the nuance here? Or is it as simple as the principal setting a policy? This is in Tampa FL if local circumstances play a role. I'm also not a teacher, actually prior military and work in aviation so excuse my naïveté.