Putting in a roundabout uses a lot more space for intersections that rarely have more than 2 cars meet. It also is a lot more costly than just adding 4 signs.
Uh, you do realize that, in situations like that in Europe, they just paint a circle on the road and put up roundabout signs, right?
So, I've never been to Europe, so I wasn't aware of that. In a few intersections near me that might work.
For most in my area though, that isn't a workable solution. The intersections are small enough that some larger cars can't do a 360 turn with out either jumping a curb or backing up and making it at least a 3-point turn. A large truck (think moving truck or school bus) would have no chance. Moving the curbs to expand the intersection requires moving utility lines and in some cases getting dangerously close to existing structures. For low-traffic roads it's just not worth it.