Honestly, it rarely makes sense.
How do you look at it what was the point of everyone following charname in the first BG? We came across much more experienced travelers pretty quickly (at least on paper).
Aside from a few characters, most characters really have no reason to listen to the player.
The same can be said for Hawke.
Why also are we in charge of the DAO when Alistair is a more experienced warden? The game is trying to explain it, but it's pretty cheap.
I could still try to cite the first act of DAI (I know there are no acts, but not important) why is Cassandra allowing us to command?
This isn't really as good of an argument as you think.
Alistair defers leadership to you quickly because he flat out tells you that he's not the leading type. Not to mention that personality-wise, his interactions with the rest of the party quickly reveals that he's really not cut out to be a leader.
Hawke is a special case. The party in that game is sort of structured as a loose collective of friends and family held together by a mutual agreement and respect for each other. It's not so much that Hawke is strictly the leader of the group in the traditional sense, more than it is everyone else deciding to help Hawke when he calls upon them for assistance with certain matters during the actual gameplay sequences, as the game takes place over the span of several years. After all, Hawke does help everyone else with their own matters upon first meeting them. When the other party members are not in the party, it is made abundantly clear that they are still handling their own affairs elsewhere in the city, like Aveline being a city guard, Isabelle doing drunken pirate things, Varric writing novels, and so on. This is one of the things I really appreciate DA2 for, and I hoped that more RPGs would try to adopt this narrative style.
Cassandra allows you to command because you have a magical glowing hand that's critical to ending the world destroying threat, and no one would accept anyone else as the leader in that kind of context. She also explains to you that she has tried to find others to lead before, and originally wanted to seek out and recruit Hawke for that exact reason, but then everything happened and you came along by sheer chance. And it's not like you're making all the decisions alone too, the rest of the Inquisition handles a lot off-screen while you and the rest of the party are out doing the dirty work.
(On a side note, if I had to describe WotR's party dynamic, it's probably an interesting mix of DA2 and DAI in execution. You have companions that are there because they're leading a faction of their own people or wield political influence that can contribute to the crusade effort such as Seelah's group of paladins, Lann's mongrelmen, and Regill's hellknights, but there are also people that are with you for their own personal reasons such as the witch Ember and druid Camellia, to name a few companions. But they all also benefit from the main character's powers, which is like the DAI anchor in terms of being a plot device.)
