Do they exist in games these days? And if so, why are they never seen?
I used to think that GM's in Everquest were cool - they added a dynamic element to the game - appearing in a zone and handing out food and drinks and such.
For a while I was very lucky to work as a guide myself on the Bristlebane server and had a blast - as well as answering petitions I good port to any location at will (with the exception of the planes which were off limits unless answering a specific petition), run zone events - assuming the guise of goblins and chasing newbies around Butcherblock Mountains, hang out in Sunset Home which was the GM and Guide only zone. I even got to conduct weddings in game.
I'd love to see this kind of dynamic interaction brought back in a game.
I haven't seen a GM in a game since Ultima Online. I tried my hand at being one on a free server and it was great, I loved helping people. Though some players can have a bit of an entitled mindset.
Guild Wars 2 has it. The ArenaNet crew do actually play the game and they have an ArenaNet logo attached to their names. If you submit a bug report it records your position and they teleport to it to recreate or observe the problem but they don't show up for face-time to troubleshoot like in UO. It's just not necessary with their game. You can move on and they can fix things while you're not watching.
They've been in the betas, though, playing and offering advice. It was quite exciting to see one in Queensdale. They can trigger dynamic events at will, I believe, but probably don't.