If you'd left them to the states, they'd have mostly been broken apart and sold off years ago, for starters. Especially when they see a budget shortfall.
This is a confusing situation then right? Taxation is the government using coercion to claim resources and labor. That's not to say it's evil, but the pool to claim resources from in a given year is fixed. So it's not clear to me it's a good thing that the federal government has a super power in that regard that states don't have. Anyway.. no particular point to be made here.