Because the hot water pipe is not the only one that goes to the shower. There is the cold water pipe also. And, if you did raise the temp of the water heater, you would have to replace all of the piping to the showerhead also to get rid of the pre-existing biofilm contamination of those pipes. That's the logic of it that I see.

I believe the concern is that the shower head holds water leaving moisture behind all the time and not able to dry out therefore will build up mold, bacteria.......if not dried out to keep it from forming bacteria. The temperature of the water helps if the shower head is kept clean or changed out every few months and a new one is installed. Some suggest buying an inexpensive shower head and change it every six months.....or not having a shower head and letting the water flow from the pipe without a shower head.