(edited to correct a typo)
Some things need to be guaranteed for every American citizen, and that can't simply be left up to the states to "experiment" with. We're talking about government here, not scrum teams.
There is virtually no way to have a consensus on what precisely needs to be guaranteed. I would personally prefer to move to a state that does not "guarantee" health insurance (I will pay for health insurance myself).
But like I said, moving isn't a liquid matter. There's quite a bit of friction involved. So states are imperfect, yes. But I still prefer 50 imperfect laboratories to 1 imperfect monolith. The potential of moving to a local maximum of ideal-fit with my tastes means a lot to me.
For what it's worth, the same rationale is why I do not believe the United States should adopt any given policy simply because it is done by a majority of other first-world countries. Policies should stand on their merits and should not be adopted simply to be aligned with other countries. I prefer diversity.
There was a consensus; we call it the Constitution. The problem is that American lawmakers today aren't able to negotiate a single law let alone a document like that.
States are not really a solution to that problem: large states have gridlock much like the feds, and small states create the patchwork of laws that prevent me from mailing you a beer but ensuring some lawyer at LargeCo will be paid to figure it out. And all states are more vulnerable to corporate capture than the feds are.
Laboratories of democracy work when we have democracies that are worth studying, and when politicians adopt fact-based policies instead of ideological ones. But we do not live in that world, and have not for at least 30 years.
Move to a better state.
Central planners think that there should be one state, the US state, where the individual "states" derive their power from the Fed. And if you think that the states should be small municipalities then that's fine too. But it's not hypocritical that municipalities derive their power from, and are restricted by, the state.
States have always been (partially) sovereign, and there is a clear trend over time of them becoming less so.
What are recent egregious examples of this in your mind? Are you talking about ObamaCare?
* Drug laws (i.e.: Colorado has "legalized" marijuana but it's still federally illegal and for now the federal government is just ignoring enforcement for now, but there's no reason, e.g. Ted Cruz would continue to do so.) The laws are based on "interstate commerce" which the federal government how power over, yet somehow it still applies to state-grown marijuana.
* A lot of stuff very close to a "war" has been done without states actually voting to go to war.
* Personally, I'd like a state to be able to experiment with a basic income guarantee and corresponding removal of minimum wage, but that wouldn't be possible because of the federal minimum wage law.
* Federal agencies like the FBI, CIA, and NSA have been growing in power lately with their FISA court rubber stamps and attempted use of All Writs.