They answer: "Because it's a good game." And while I can't disagree, I just find it amazing that people might still be playing this.
It saddens me that these worlds are becoming steadily forgotten, but it also makes them all the more exciting to explore.
It's curious how the impulse to "play" the game falls away without the frantic shots of your team-mates urging you on, without the burgeoning newness of the game waiting to be discovered and without a brand new ranking system pressing you to try your best. In old abandoned spaces such as this, chance encounters with other players turn you both into wayfarers with a mutual love for the same territory. You're more inclined to treat other players as people rather than merely as targets.
I come to realize that I rarely reminisce about gameplay specifically; recalling past frags or victories evokes very little in me. It's the beauty of these game worlds - the wastelands of Mar Sara, the still lakes of Na Pali and the brooding back alleys of Hell's Kitchen - that brings syrupy tears of nostalgia to my eye. I'm reminded of Tale of Tales' assertion that "The game structure of rules and competition stands in the way of expressiveness." I presume that they meant this foremost as a design principle, but it's possible to understand the truth of it when rules and competition disappear through the abandonment of these old online worlds. This mass departure of players encourages a repurposing of the game's space towards the simpler ends of exploration and aesthetic appreciation. My experience of the Abandoned City felt more atmospheric, richer and nostalgic than it ever could have with wave after wave of NSF and UNATCO troops slaughtering their way through it. This is not just true of Deus Ex, but of so many of my old games.
When I recently loaded up Half-Life Deathmatch Classic I was shocked at what I found: 2 out of 20 servers were populated by a grand total of 7 players. Joining them is like stumbling in on some odd, ancient ritual. When you ask them why they're playing such an old game, they often fail to see what's so strange about it. It saddens me that these worlds are becoming steadily forgotten, but it also makes them all the more exciting to explore. It feels transgressive. I like the idea that another player like myself might log on and wonder what on earth I am doing as I slowly wander around admiring those gritty old textures.