When you are in the office, the company has almost all your attention. There is almost nothing to distract you from work.
When you are at home, there are a million things to distract you. There's laundry that needs to be done, the kitchen cleaned, kids that want to play, there's a full fridge with stuff to make a sandwich, a TV that you can turn on, etc.
When you are forced to go to the office, you are forced to take care of everything you need to do outside of working hours. You have to make sure you have child care available, you have to make appointments for service technicians in your free time, or ask someone else to take care of it.
Seen like this, I think it's obvious remote work is a perk for the employee. Employee gets more flexibility, but employer gets less attention. So I understand why employers would pay more for people who come to work on site.