The assumption here is that there’s a symmetry between private parties wasting their own money and the government wasting the public’s.

I think the assumption is that both private and government try to minimize waste as much as possible, and the government is just better at it.

you're wrong about the private waste, I'm angry about it also but I can't do anything about that. But as a tax payer in the top 2% in the US I am justified in my anger towards the US govt wasting tax payer dollars shooting AIM-9x in F-22s to blow up balloons.

Was there a more cost effective way to shoot down the balloon? Is there any actual competition in the missile industry?

I read the story about the F-18 shooting down a balloon with its gun, but it's also expensive to fly and maintain such a jet.

…not shooting it down in the first place.

If a balloon really is operated by someone attempting to spy on sensitive military installations, it makes financial sense to shoot it down. It's cheaper to spend 400k on a missile than to have your adversary learn something they shouldn't.

If the federal budget is $2T and if you paid $100K of that in taxes, your contribution to a $400k wasted missile is two cents. I might suggest that two cents does not justify very much anger.

why can’t an enterprise be more efficient if profit is their motive?

probably because in many enterprises people are incentivized to optimize for personal profit (pay grade, promotion, increasing team size, launching new (often unnecessary) features/products etc) which leads to waste and bureaucracies... iow optimizing in the small which leads to the opposite in the large.