The USA has a huge military budget because:
 it values its freedom and independence
 it values its allies and trade routes
 it costs the USA a lot more to get any measure of military might since it does not have a conscripted military and its materiel is not made by government suppliers.
The USA has a huge military budget because:
 it has whipped it's population, and that of a significant portion of the worlds population, into a frenzy of fear and paranoia, by maintaining a permanent war status since the end of WWII. Whether hot or cold, the USA has waged war against non-enemies, aggressively supporting brutal dictatorships around, killing millions in defence of "freedom and democracy" around the world. The USA as "protector" of the "free world" has been holding the human species hostage to the possibility of imminent extinction via nuclear weapons for nearly 70 years now. This in the name of valuing its freedom and independence.
 it has decided that the best social control mechanism for social pacification is to provide the lowest possible cost for goods and promoted consumerism as ersatz status symbols for a largely disenfranchised permanent underclass. In order to secure such cheap access to goods, the USA must dominate all world trade and be able to dictate prices for resources, resource extraction, production and distribution. This in the name of valuing its allies and trade routes.
 it spends a lot less money for the size of our military than any other country would for a similarly sized and scaled military because 18 year-old's with no prospects always form a cheap labor pool( "sold" as in soldier, comes from Roman Latin, daily wage-worker), particularly if one can convince them that they are serving their country men in the name of noble goals and values like "freedom and democracy". And because military production in the USA has always been "dual-purpose", a civilian feel-good, while producing weapons of mass destruction.
The USA has co-opted a tremendously large section of it's so-called private market for dual-purpose: production of disposable goods, while supplying the worlds largest military with an endless production line of "goods", of which no good can ever come. Most major manufacturing firms in the USA would not be economically viable if it were not for this arrangement. Boeing could never make it solely producing commercial air-liners, the market for such is not large enough, but if the Pentagon needs x number of fighter jets, missiles and rockets every year, which must be constantly restocked, due to usage in wars or due to degradation from not ever being used, they can show a profit for their civilian production lines. The same is, and has always been, true for GM, Ford, GE, etc.
The USA has not, historically, HAD a "health service" because we traditionally left health to the people themselves, the private sectore, and communities and states (The US Constitution says nothing about healthcare, and it explicitly says that anything it does not assign to the federal government belongs to the people and to the states)
The USA has not, historically, HAD a health service, because immiseration is a constitutive part of maintaining an exploitable permanent underclass. During the first 250 years of American history, surplus value, ie. profit, was made primarily by either a) stealing the indigenous peoples lands or b) exploiting "free labor", ie. slavery. Following the civil war profit has largely been made by exploiting those born into inter-generational poverty, both here and around the world. The majority of Europeans who immigrated to the USA during the first 150 years of colonization were debt-prisoners, ie. the then european permanent underclass. The USA has proudly cultivated cultural identities for the permanent underclass, going back almost 350 years, nowadays we call them "rednecks", the "n-word", and "mehicans".
The constitution, written for the most part by by the 18th century 1%, actually went to lengths to prevent the existence of a standing army. The USA has been violating it's own constitution every day for nearly 70 years now by maintaining the largest scale military ever seen in world history in direct opposition to our founders intents. With a media hell-bent on scaring the bejesus out of everyone 24/7, covert intelligence services, dedicated primarily to propagating domestic propaganda(be scared, be very scared), and brutal police forces, who routinely terrorize, denigrate and humiliate our permanent underclass, we have held our population in line through domestic terror, knee-jerk reactionary patriotism, dog-whistle ethnic discrimination, and cultural envy as a function of permanently denying the permanent underclass of the USA access to the "freedoms" and "democracy", which supposedly are those things that make America all so great.