Because otherwise there is no plot for you to follow?
Because it's poor writing, you mean.
What if you don't need to side with the tranny? What if you can betray, depose or just murder them in the first act? What if there are bigger faction plays that can be made that don't involve being some gay orc's lapdog?
Gosh, that sounds like something that's actually interesting. Better avoid it and stick in another dozen trash mobs instead.
As a Lich, you challenge the Goddess of Death herself blahblah
Who cares? Why do I care about being the god of death in an utterly gay and boring world? To destroy it? I could just not play the game and save myself the trouble.
It's not only the gameplay that's supposed to provide a masturbatory power-fantasy experience, the narrative does it as well.
No, again, I don't care about being the king of a shithole that I don't even like. The worldbuilding is absolute ass in this game, so none of it matters to me.
OK, let's assume you're the one writing WotR. How would you work your way into your character being made the Leader of the Crusade if your character abandoned the last piece of the city not occupied by Demons and left some of the Queen's most loyal servants to get gangraped by Cultists and Demons?
Assuming that we have to lead a Crusade at all. Assuming we can't join one of the invading factions early on or a neutral faction that seeks to play both sides against each other.
And as for the Queen's most loyal servants, one of their group betrayed the city at the start of the game iirc, and the tranny and the orc were too stupid to notice a demon invasion that was happening right under their noses. They're utter failures at their jobs and the Queen would be an absolute retard to miss them, let alone to keep them on her payroll after such a massive screwup.
"Why do I care about that annoying brat Imoen and some crazy Wizard? Why can't I just return to Baldur's Gate and chill instead of having to find my way into Spellhold?"
Oof. Ruined your own argument.
BG2 offers multiple reasons to go to Spellhold: to save Imoen, to take revenge on Irenicus, to use Irenicus' knowledge to gain power, to stop whatever evil he's planning and keep him from murdering anyone else. It's actually bothers to consider player motivations, and the dreams, while foreshadowing, also help to direct the player to choose from one of these motives.
And you're not forced to do any of these. You can just run around and do sidequests until you're bored or you run out of things to do. You can just let the two of them rot for 20 years if you want. There's no hard timer. You aren't stuck in Athkatla until you do.
It's the exact opposite of Wrath.