I can only assume it is because having a reporter 'on site' adds a level of realism and gravitas that would be absent if they simply reported from a studio, which would make every report look the same.
I tend not to notice when they do it for news pieces, but as regulars on here will know, every winter I post about the utter pointlessness of having some poor sap standing outside in a Northface jacket freezing their extremities off, to tell me it's snowing.
I was around in 1963, I KNOW WHAT SNOW LOOKS LIKE!!!!!!
Just tell me it's snowing, I'll understand!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!