Managers of Reddit, why do you send out blanket emails to everyone instead of taking it up with those members of staff that are the problem?

I work in a service where some workers just don't do enough. We all know who they are. Managers know who they are. Instead of talking to these staff and dealing with the problem, managers send out an all staff email saying that they are aware that some staff don't do enough and that it isn't good enough. The consciencious staff think that they are the problem and try to do more and end up burning out, the lazy staff think that the email is aimed at someone else and do nothing. Rinse and repeat, again and again. Can anyone explain to me why managers aren't managing?