People who believe (in varying degrees) that 1 USDT == 1 USD.
In countries with strict capital controls (China, Russia, etc), USD is inaccessible to most people. And the fact that USDT has been trading at 1 USD for the most part is the proof that most of its users either believe 1 USDT == 1 USD, or they simply don't care.
In these countries, you have large amount of OTC desks due to lack of access to exchanges. Those OTC desks have to provide liquidity for large amount of trading activities (very large but I wouldn't think it's anywhere close to the 13b USDT that was printed in the last 6 months), and therefore they have to hold sufficient amount of USDT for that purpose.
Once you have a group of people (miners, traders, etc.), USDT could act as as a token valued at 1 USD among those people, and at any given point, there would be a significant amount of people holding large amount of Tethers. After all, the Tether "FUD" has been ongoing for years and nothing bad seems to have happened, right?