Why not? Taxes are just enough to pay the interest on the national debt. It's throwing good money after bad. Starving DC can only be good.
I support a debt default. It might surprise you that a minarchist would take that position. My position is a little unusual in that I think government for the core three functions are justified, both Biblically and to a lesser extent, logically and pragmatically. But I consider any government action beyond that to be a lesser or greater degree of "Evil." So I'm that rare "radical" minarchist.
I also don't necessarily think we should have a Federal government. Government =/= Federal government. I support radically decentralizing, but not totally eliminating, government.
As for why not? Well, defense is the biggest reason. Honestly, if the USA stops being an empire, someone will take our place, and we need to be able to defend ourselves against them. I'm very pro-gun, but I don't think a bunch of people with machine guns alone will be able to effectively defend the country. I don't really support a standing army per say, but I support a government who can quickly organize one when needed. (Not with the draft, for the record.) Courts and police is another reason. I don't think a free market in justice is a great idea. There are some cases where I'm fine with free market justice competing with the government. In a discussion in school today about some states fighting against Sharia Law, I suggested that if two Muslims or whoever want to solve their disputes through Sharia Law, its illegitimate to stop them from doing so. However, in the vast majority of cases, where the two people are not going to agree on some such thing, I think we need a court that respects the non-aggression principle, not one that simply sides with the wealthy and powerful. Which may be a pipe dream, but what isn't in politics? Anarcho-capitalism is just as unlikely.
That said, the theoretical stuff isn't really what gives me a passion for politics. I'm passionate about politics because American wars kill hundreds of thousands of innocents. I'm passionate about politics because the drug war has given us the largest prison population in the world. I'm passionate about politcs because they think they can watch me whenever they want. I'm passionate about politics because government bureacrats are trying to destroy our second amendment rights. I'm passionate about politcs because the unborn are being slaughtered and nine men on a Supreme Court are preventing our state governments from doing anything about it. I'm passionate about politics because we have more laws than any other country. I'm passionate about politcs because the entitlements programs are destroying the wealth of our country and robbing my generation to support a ponzi scheme that we can never benefit from. I'm passionate about politics because government takes half of what we own, and claims they have a right to do it. I'm passionate about politics because they think they can control what I can say and where I can say it. I'm passionate about politcs because they can take our property, any amount they like, and the burden is on us to prove our own innocence. Exc.
The philosophical debate between minarchists and anarchists is not what makes me passionate about politcs. Heck, little issues like roads and the like don't make me passionate about politics. I do care, a lot, about privatizing the schools, but that's more because of the indoctrination aspect than the money. If we decided everyone should have a primary education, I honestly wouldn't have a huge issue with it. I wouldn't agree, but that would be a minor issue that isn't really that big a concern for me.
Walter Block breaks libertarians down into anarchists, minarchists, and classical liberals. I'm technically in the minarchist camp, but its not really the slight differences between the different grades of medium-core to ultra-hardcore libertarianism that really get me passionate about politics, its the real, concrete issues that I described above. I think anarchism is a little too idealistic and classical libertalism is a little too pragmatic, but I'd vote for either of them into political office.
Even people like Rand Paul and Gary Johnson, who clearly fall outside the category of "libertarian" as Block draws them, are still mostly our allies against a Leviathan state, and they give a LOT more concessions than any real libertarian would. You've got to realize the big fight here is between limited government advocates (The "No government" advocates are a very small force, but I would consider them to fit under this group) and unlimited government advocates. If we make it about anarchists vs minarchists, or even libertarians vs constitutionalists, I think we're screwed.
TLDR: Because I think its a necessary evil, but more importantly, because total abolition of government is not what makes me passionate about political issues.
I will note that the issues I listed above are not deliberately in any particular order, and may be incomplete. Do not necessarily take silence on a particular issue to be equivalent to being OK with it. If its not clear, please ask first.