I've always thought this as well. They can more effectively control the population and also get "dirt" on anyone they want. There's pretty much no one on this planet who hasn't said or done something they aren't proud of, don't want public, don't want their spouse to know, is mildly illegal (but which could then be vigorously prosecuted once they are a target), etc. etc.
I think it's for the Donald Trump scenario - if an "outsider" tries to enter the political arena to gain power they can crush that person using all the available tools in their arsenal. Regardless of how you feel about Trump, he was able to combat that because he was already rich and famous. No regular human being could rise to power without the approval of the existing political elite. Mass surveillance is just one tool they use to maintain that power.
You basically outline why that's an unsatisfying answer. It didn't prevent Trump from entering the political arena. The "Access Hollywood" tape surfaced a month before the election, why not during the primaries, when he was more vulnerable? Does anyone really believe that someone with purportedly the full power of the US intelligence community couldn't find more on-the-record dirt on him, or his senior staff? What about the stuff in the Russian dossier (the golden shower stuff)? Again, appeared days before the election, too late to do proper damage, and if that's the best the US intelligence community can do, they're mind-bogglingly inept.
Assuming the conspiracy theory that the US IC "effectively control the population" is true, the only rational explanation is that they wanted Trump as president, not the opposite.
