The monster has the most control over the pace of the game. It is faster than the hunters, it can choose when it wants to fight. To stop it just constantly running away and hiding, the game has to have a way of forcing the monster to try to win.
Nothing nefarious, no great conspiracy to try to sap the souls of monster main players, just basic rules that ensure everything is guided to the intended end point.
Edit: Canonically it might be worth you visiting the Evacuation campaign, the cut scenes describe that while you’re fighting monsters there are usually colonists or soldiers working on something that will take some time to complete, so you either need to delay the monster or kill it so they have time to do what they need to do.