It just takes a shit-tonne of capital, and it helps to have a functioning internal economy and flexibility to be able to defend against malicious agents.
I suspect there are other legal issues with creating an alternative currency in the us though, which is why these aren't currencies but securities. Which is still fine! A tethered coin is conceptually _similar_ to a 0-yield bond.
Presenting it as being "safe" without it actually being so is the problem here.