A huge part of the charm of TTRPGs is playing a non-digital game face to face with other humans. Even if we had AI that was as entertaining as a skilled human DM (unlikely to be possible IMHO), the human might end up being preferable since hanging out with humans is one of the goals of this particular pursuit.
Plus, we already have (weak) AI DMs. They're called video game RPGs. You're the player (as are many others in the case of an MMO), the game itself is the GM. They do many things better than a human could (handle lots of background math, present gorgeously rendered environments, have professionally voice acted NPCs, have skill based real time combat), but the audiences don't entirely overlap because TTRPG players are seeking the freedom of choice and limitless outcome that video games cannot really match.
Plus, we already have (weak) AI DMs. They're called video game RPGs. You're the player (as are many others in the case of an MMO), the game itself is the GM. They do many things better than a human could (handle lots of background math, present gorgeously rendered environments, have professionally voice acted NPCs, have skill based real time combat), but the audiences don't entirely overlap because TTRPG players are seeking the freedom of choice and limitless outcome that video games cannot really match.