11% of the ETF market is synthetic, and it is effectively illegal to launch one in the US now.
So it appeaers that its a) not half the ETFs and b) the regulator aggrees.
ETFs, hold the thing they are convertable to and are audited. the equivalent would be the stable coin having $1 in a bank account for every stable dollar they mint ... and be willing to be audited.
The SPX etf holds the actual constituent shares 1:1 with what they are redeemable as so it is a terrible example from your side. It doesnt hold a bunch of chinese equities, or discount miami property on a punt that it will do better than their underlying asset.
Stable coins have (not Dao as I agree they have a very nice fixed protocol) taken their 1:1 dollar backing, punted it on crypto, and when it went up and they were overcollateralised - paid themselves the excess personally. When crypto has gone down in value closed their eyes and eventually gone bust.