There is one very good reason to not do it every time: game flow and mental fatigue. We all know how important it is to keep it as simple as possible when things are getting tense and you're getting tired. By calling a ball every time, you're adding the following steps
- Identifying the possible balls that might get bumped
- Identifying which pockets those balls might get bumped into
- Choosing between the options to determine the best one
- Calling the pocket
- Making sure the ref or your opponent heard you
It's super minor, but it's definitely a drag. Yes, you can short circuit the process a bit and just choose the first one that comes to mind or not worry about whether or not your opponent heard you, but it's human nature to consider all possibilities once you start.
I think the way pros play 10 ball is pretty optimal: just call a ball whenever they think there's a chance they'll accidentally make a ball. It just happens that pros are humans too and sometimes you misjudge the situation.