The big difference between DF and a challenge-driven AI or DM is that if the AI decides it needs to send a siege of 500 goblins to challenge you, it just dips into the bit bucket and pulls out enough 0s and 1s to create 500 goblins, and throws them at you. It's a black box with your fort (or whatever you are running) on the inside, and the black on the outside. The AI can just create anything it wants to keep you challenged without having to worry about how that affects the rest of the world, since there isn't a "rest of the world".
DF, on the other hand, models the entire world. It tracks how many goblins are in that dark tower that's been attacking you. After sending 300 goblins at you last year, the tower might only have 400 goblins left, so it's simply can't send 500 this year.
This may not be the game that you want, but it's the game that Toady is building. He's building a world simulator, not a tower defense game. Now, hopefully he'll realize that the game isn't as interesting (or as ‼FUN‼) as it used to be and do something to give the sieges a little more umph. He knows that he makes his living on player donations, and if the players all get bored and quit donating, he might have to get a regular job. But he also has his vision of what he wants to make, and I don't think he'll give that up to satisfy people who want something different. I respect him a lot for what he's doing, and I hope he's able to figure out a way to both create what he wants and create something that users want. It's like trying to walk a 1 tile wide path over a 20 z-level pit, with spiked wooden balls swinging at you from time to time, it's tough to keep everything balanced.
Maybe just doing something to increase goblin population would help things. That could be done by making them breed faster, making them mature faster, or making them train faster. I suspect that any of those could be modded in as things currently stand.
He has plans at some point in the future to do a major overhaul of armies and sieges, but I'm not sure how high that is on his list, and could take years.