I bet when the billionaires of today talk to the experts who could do this stuff, they are told that it's not been tested on humans, so they don't know how to do it in a way that would allow anyone to bring you back later. I mean, essentially it seems like you'd try a bunch of stuff until it works. But we can't, because humans have rights, and nobody would volunteer to be frozen for experiments (probably), and even if they did, I think there are rules against that sort of thing. And if you just throw a naked guy in a freezer, then I'm pretty sure he will die
It's possible that evil billionaires are working on this right now, or rather that evil scientists are working on this on behalf of the evil billionaires, in secret underground labs. The problem is that we'll never find out about it, or be able to partake in the technology. I suppose it's also possible that some world governments are working on this, but in that case I doubt they would tell us if they succeed either. If you're getting frozen, you probably want your location to be super secret to most people
Maybe NASA or SpaceX or some sort of related space company will one day figure it out with frogs or something. And then some crazy guy jumps in the chamber and they revive him a week later and there you go, human testing