The US spending money to fund Ukraine has got to be close to the best defense investment ever.

The US got a direct proxy war that appears to be breaking Russia and didn't spend the life of a single US soldier. It collapsed Wagner which was a threat to all manner of developing nations. It fixed NATO unity practically overnight. And China got a glimpse of what would happen if they invaded Taiwan and probably set back the plans of Comrade Pooh by a decade or more.

By contrast, the US blew how much in Afghanistan and got not a damn thing?

We elect the government to use the tax money wisely.

Many find that, per dollar, nothing has been more effective in decades in strengthening nato allies, weakening a totalitarian state that routinely assassinates outside its borders, and stress testing a military supply chain.

Eisenhower noted in his account, Crusade in Europe, that (paraphrasing from memory) that "Few really understand the relationship between America's safety and prosperity and events far away in Europe."

Is this something that has been forgotten?

The US government considers supporting Ukraine as beneficial to US, why shouldn't they fund it?

They shouldn't. NGOs can't buy weapons for the military. Ukrainians would be fighting US soldiers a couple of decades from now if Russia were able conquer Ukraine in a few weeks as they were planning.

so would one get to opt out of funding social security and all of the other social services that the government spend money on too?

I'd be more than glad to send them tanks and other munitions if I had any.