Because people holding them have the expectation that they are each worth a dollar. So if the price goes down to a few pennies, they will lose all of their money. The direct effect of this will be to wipe out confidence in the entire market, since tether is a huge part of the system. Another effect will be for the fake dollar printing press (tether) to be switched off. Removing $100M dollars a day fake money that raises all crypto prices. The drop in prices of crypto could be startling.
There's lots on google and on reddit to explain these issues more in detail.
Hope this helps.