Because you can't transmit cash to crypto exchanges? Also, Tether is the oldest and most rock solid stablecoin. Maybe supported by criminals but they've never failed to redeem
I personally don't believe any of these things make the case for holding a stable coin.
Remember the coin is pegged to a real thing, so it can't outperform, but it is at risk of total collapse. The only way for the "cash" you hold to collapse is for the entire banking system to collapse. In which case, you're still better off holding BTC.