Unfortunately, in America, at least, that would be likely to lead to a lot of lawsuits—lawsuits that should be shot down under the SLAPP category, but it takes enough money and time just to respond to a lawsuit that many people would be unwilling to take the risk by posting anything.
It's a multiparty problem. I really wouldn't want to post negative experiences of companies under my and their real name while still looking for jobs elsewhere.
Glassdoor could only possibly be accurate because it was anonymous. Of course, that also makes it easier to fake.