That's easy to understand USDT isn't safe so why don't traders see it?
Why were people still holding Luna while this was crashing and calling it FUD?
Why are people trusting Binance #safu when it's clear they don't have the amounts needed to reimburse a bank run?
Why are some trusting BUSD when they don't even know where the entity behind BUSD is located?
Why are people still holding their coins on CEX even if they don't daily trade and why are some still risking assets for 5% apy?
or, and let's go back in time
Why were some calling MtGox crash FUD and were still trying to send money to that exchange to get cheaper coins?
Each person has their own way of thinking, each one will rust to believe in something and completely deny something else, from religion to the politicians they vote for to the coins they invest, so there will not be a moment without at least somebody claiming that tether is safe, that is backed by something and that we who doubt this are just FUD spreaders and we're doing so because we work for the competition.
The same reason people continue to use all manner of unsafe and risky platforms - greed.
Besides greed there is also another reason, some simply don't want to accept they are wrong, so after claiming for years that UST or Binance or Coinbase is safe and that nothing can happen they don't like to admit what they are doing is risky, it has worked in the past, till now everyone who warned was wrong, so they feel like they are right and they will be always right and the rest are simply just envious fear mongers.