They don't have to be remote. Hence the term "digital tower".
In theory, I guess, you could have HUDs with the same information over a traditional tower's glass windows. But then, depending on where you are looking at it from your perspective is skewed.
A solution for that would be to use cameras and display eqpt and no windows.
From there it's a question of cable length and susceptibility.
Then you hit additional opportunities to reduce costs, both as Gonzo mentioned for the airport and for potential pooling of ATCOs at a remote centre.
Remote facilities will not suit all airports. Digital towers, I suspect, would provide benefits for most.