I think the reason as to why The Blight is constantly underground is quite sad and depressing( A tin foil hat theory)

So we now know after Veilguard that the Blight is Titan's dreams driven mad from imprisonment and torture, I think I understand why the Blight is a constant presence underground and frankly it is sad.

Because what lies underground in Thedas? Titans' Physical bodies. In a way, the Blight that escaped their prison have been trying to connect with their physical bodies. Those dreams are trying to reconnect with their owners. But as we have seen with the Wellspring in Descent DLC and Lyrium Sanctum in Harding's quest, a Titan's Physical body literally repels the Blight.

So what is happening, then? The Blight, the corrupted Titan's dreams are trying to connect with their Physical bodies and the Titan's bodies rejecting their own corrupted dreams. And it is going on for eternity.

SO what is next closest thing to Titans that the corrupted Dreams can connect to? The Dwarves, the children of Titans, to whom also those Dreams belonged to. So the Blight then tries to connect with the Dwarves, only to not realize that they are so corrupted and deranged that they will only drive the children of their owners to madness and death and they do just that, destroying the entire Dwarven Civilization and ensuring cities like Orzammar and Kal Sharok under constant threat from the Blight.

It is sad, because after so long of imprisonment, the Blight is only trying to connect with its Physical bodies, only to not realize that they are so corrupted and dangerous, that either they drive the Titan's children to madness and death or get rejected by the Titans and thus they lash out more and further spread that destruction. It feels like a tragedy.