Many, but not all. There have been several studio monitor's I've used had separate left and right. NS-10's, Monitor 1's, some JBLs, Samson, KRKs... I will admit, nearfield monitor's tend to be a lot more homogenized now than they did when I was doing studio work.
Given our readership, I hope you'll give me some grace if I applied the concept of "powered speaker" to these studio monitors in the review. I'm trying to meld the words together, if that's possible.
However, I fear I simply insulted all the studio engineers...!