Because first of all, without Hawke they wouldn't have had any investors, so the expedition wouldn't even have started in the first place. But if we go to the land of make-believe, then lets say the expedition did set out with just some random mooks of mecenaries. Then they would have been slaughtered to a man, by the Darkspawn, dragons, Profane, or demons, take your pick. This would result in Varric not playing any future role in Kirkwall. Which means that even IF Varric somehow succeeded in helping out Anders in his little quest, then he wouldn't be there to help Anders out with keeping the Templars off of him. WHich would again result in Anders capture.
Then there is also the whole Qunari business which again without Hawke, would have resulted in Kirkwall being burned to the ground by the Qunari, again probably resulting in Anders death or capture, but much more importantly making Anders' planned terror bombing of the Chantry moot, since it wouldn't be in existance anyway.
Bottom line: No Hawke --> No bombing --> No escalation.
You're basing that argument on the idea that Hawke - and Hawke alone, never mind the companions who did three quarters of the work and would still be available for Varric and anyone else to collect had Hawke not been there - was the only person in Kirkwall capable of scraping together fifty sovereigns. You're already in the land of make-belief.
And again, plenty of skilled mercenaries(you later meet bandits in the street who are at least as powerful as Hawke is at the start of the expedition) and plenty of desperate apostates who could do the same job. I played the exact same game as you, and not once during the expedition did my Hawke personally do anything that nobody else could or would have done to make it succeed, or even make the defining difference at any critical point.
No evidence whatsoever that the expedition would have failed if Varric had brought anyone but Hawke in particular as the fourth member of the party. And Varric DID help Anders out in his little quest, he put him in contact with a few mercenaries for backup, which just so happened to include Hawke, and again, I never saw Hawke personally do anything that another mercenary or apostate, or any other party member, for that matter, couldn't have done in his place.
And yes, let's look at the Qunari invasion. What difference did Hawke personally make there? He killed the Arishok in single combat. After which Meredith promptly appeared with Orsino and four templars after having secured the rest of the city. The duel + negotiations between Hawke and the Arishok in that scene take a total of 4-5 minutes on average, and it doesn't look like the Arishok is actually doing anything when we enter, aside from monologuing to the nobles.
So, if Hawke and co. hadn't been there, or if Hawke had lost, they would have arrived and either found things in pretty much the exact state we did, or found some of the Qunari dead and the Arishok still alive but caught inside the palace where they could easily box him in and wait for reinforcements.
Either way, the only difference Hawke's presence makes is how many templars die in the ensuing battle before the Qunari are suppressed, and whether or not there is a person called Hawke who deserves to be called Champion at the end of it, which in and of itself changes the apparent narrative of the events dramatically but doesn't mean squat for the events themselves.
"I was there sometimes and people say I did a really good job." doesn't equal "So obviously the city would have burnt down if I hadn't been there. And everybody I knew would have died. Which is obviously why it's completely thanks to me that my acquaintance who would later have a much more dramatic effect on events was able to do that. Never mind the guy himself, or the people whom my existence must have allowed to directly help him. It all comes back to me doing a good job."
I can see why the dumb masses could credit Hawke with a lot of the events of the game in-universe, given his status as Champion and possible magehood, but that's not how the actual events played out when you look at them objectively. He's certainly no more responsible for the results than Varric is, which might actually be why Varric himself talks Hawke up to anyone who'll listen. A figurehead, of sorts.