Basically, because the military got a massive budget in WWII and Americans just got used to it because slaughtering the Nazis was the only thing that can convince Americans to buy into that level of welfare.

Now it's mostly a jobs program for poor people plus pork for politicians to throw at their favored contractors/companies. Can't really be eliminated without political suicide because too many mouths are fed off of it and will make it their mission every waking moment to damn anyone who tries to do it.

Since prevention is a lot cheaper than cure we're trying to avoid the same mistake with other things rather than commit political supuku on things that already exist.