1. because UHSS is sensitive to heat, and robots are much more accurate in how they heat than Jimmy with a tig torch, and they were programmed by a process engineer, where as Jimmy welds until 'it looks good'.
2. welding may compromise anti-corrosive treatments on the inside of inaccessible cavities, which can lead to corrosion issues
e.g. https://rts.i-car.com/crn-24.html
A crappy shop will certainly just weld panels in without any regard for materials engineering, but it results in a crappy repair.