The low number of U.S. causalities keep outrage to a minimum. Lots of people probably don't even know someone who's been wounded in combat, let alone killed.
The monetary cost is just invisible, it's not like a lot of citizens are really tracking the budget. Those who don't, but claim to care about the national debt will usually go after social programs first because the idea of welfare queens and people they just don't like using social programs is magnified into a serious huge problem that if we could just solve that we could afford everything else easily...
It's the equivalent of screaming about someone dropping a penny but lighting a %100 bill on fire to get everyone's attention for the tirade.
And while many U.S. citizens know the U.S. has a large and powerful military I don't think they really have the idea of the scale it really is compared to the rest of the world. At which point I like to point out that there's 20 active aircraft carriers in the world, the U.S. operates 10. We also operate 10 escort carriers that are smaller, but still larger than half the other aircraft carriers operated by other countries. Just as a single example. And it's not like we've just run amok only with aircraft carriers.
For one, even if you take the 6tn total cost for these two wars you're talking about 330bn a year for 17 years. A staggering sum to be certain, but the USG's expenditures measure in the trillions annually.
For another, our immense military has almost certainly prevented regional conflicts (say Saudi Arabia vs Iran, China vs the rest of the South Pacific) and has allowed other nations to bring up their economies without having to worry about military spending (Japan, Germany).
The net positives from a more peaceful world are very hard to measure but a lot of it is having such a large dog in the fight.
More equality of armies means more regional conflicts.
Finally as a measure of GDP US military expenditures are still less than several countries.
You can point to our many follies because of course you can, that's simple and easy, and I'm not trying to defend them. But to say we've run amok would imply that they've done no good and I don't believe that to be the case.
Like it or not under US hegemony the world's been safer and more prosperous than its ever been and a tremendous number of people have been lifted out of poverty globally.
Is that all because of US hegemony? Maybe not but I'd be hard pressed to see a clear argument for why the world's defining super power would not have had some measurable role in the fulfillment of those measures.
If we were to have war taxes, directly paying for active wars, I think it would be quite a different story. But Americans who vote have gotten exactly what they asked for (in the aggregate): wars they do not have to pay for, wars they do not have to volunteer to fight. It's hired out to others, with borrowed money, doesn't affect me.
Understand, this is perhaps a bizarre defense of the stereotypically ignorant average American, that American foreign policy does not directly affect Americans. The U.S. government has a longer reach and effect globally, than domestically. And most Americans don't know what that looks like.
This is the huge elephant in the room.
The USA needs a large jobs program and a massive amount of education funding, especially for rural youth who do not either study a trade or go to college immediately after high school.
Providing these things directly with no strings -- or even as compensation for civilian service -- is wildly unpopular. The military is an absolutely massive back-door social program.