Because while you're commuting, that's effectively "company time" you're not getting paid for. If you work 8 hours a day and your commute is half an hour each way, then you're taking 9, not 8 hours a day out of your schedule for work. That's an extra ~250 hours a year you're taking out of your own time for work, whereas with an "instant commute" WFH, the moment you logoff becomes personal time again.