The absolute amount of logistics required for a show is tremendous, and if you look at a tour schedule you'll often see that it's absolutely packed - no time to add in extra shows if others sell out.
This is why the artists will have fan clubs and such, it can greatly help them determine what they could sell and how many shows. Sometimes you can switch to another venue in the same city, but that only works when they're both owned by the same company, usually. And it other cases you can open up more "seating" but that really only works when playing to stadiums.
I noticed when Rammstein announced their European tour recently, there was one show listed in Denmark. On the day tickets went on sale, there was a second date at the same stadium.
This is why the artists will have fan clubs and such, it can greatly help them determine what they could sell and how many shows. Sometimes you can switch to another venue in the same city, but that only works when they're both owned by the same company, usually. And it other cases you can open up more "seating" but that really only works when playing to stadiums.