What many crypto 'traders' will have done is put whatever gain they made straight back in and many will have subsequently lost it. The moment you realise any gain is when it's taxable and you need to secure that there and then. If you don't then you can wind up with nothing and a tax bill that still runs in millions of dollars. I've seen a few cases of this now.

In this case at least they have the money to meet the debt. Many won't.

I'm not sure whether crypto to crypto trades were taxable that way in 2017. That has been clarified and they are now in the US.