Probably because OTHER systems do work for other people. Ours isn't necessarily superior. It works for us and we know no other way. Other countries have been using other systems and they work just as well. Hey, I know this may seem like heresy to you, but a king will work too, only, that is, if he's a, um, benign despot. Lol. Please don't slam me back on this. I wouldn't trade our system for any other.
The key is to have democratic free elections for those coalitions and parliaments. If the people freely CHOOSE their government, then it IS their choice. One can ask no more of them. It DOESN'T have to be a clone of our system to be good, worthy, useful and representative.