Because when the game first started, it was the best place to get decent gear. Unfortunately I hate PvP, especially gear-loss PvP.
I guess it's technically not quite gear-loss PvP. You don't lose anything you currently own, all you'd potentially lose is any recent loot you've picked up that you haven't yet extracted. It's down to the player to either extract ASAP, or continue on for further loot.
From what i've found with TD1, the DZ provides very good loot for early-mid game weapons/armor/mods, once i hit end game, i found the challenging missions were giving out much better loot, and with no risk of losing them.
I actually think it's a great aspect to the game, there's very much a risk versus reward element of play there. With no requirements to go in there, it only benefits those willing to take a bit of a risk. There's something quite adrenaline pumping about bumping into other agents and not knowing whether they'll turn around and kill you, or team up and both work on getting some good loot out of it.