How would they enforce it?
So are many other kinds of financial transactions, there hasn't been a real problem enforcing that. That's basically the IRS's job.
I was talking about unregulated securities offerings, where I'd say cryptocurrency DOES help. However, I see OP invoked the IRS for some reason- I agree cryptocurrency doesn't help much for tax evasion.
You can have 10 million USD in crypto and live a normal life: pay rent, drive a usual car, etc. Nobody will notice you, not even the IRS.
If you want to move your funds in another country, you simply have to move yourself and spend them there. You can't do that with fiat.
...or you could just pay 20% (or whatever) cap gains taxes and immediately use your 8 million USD in the US- That seems like the preferable option.
(And even then, banks can be touchy enough that you still can't get any ActualMoney out of Bitfinex.)
2. Unless you coordinate internationally, the nations with less regulation will launch successful businesses on it and dominate the market - imagine US having some crazy internet regulations in 1996, and other nations not having them. We'd be having this discussion now not on Hacker News, but on Новости хакера, or on 해커 뉴스. And US economy would be a few (dosen?) trillion dollars poorer.