Real banks practice fractional-reserve banking, where they are required to hold a percentage of assets as cash or central bank deposits. It's well-regulated and diversified. It's audited. Individual account holders' deposits are insured (FDIC in the US). This is all well-known and understood and reliable. Runs can still happen, shady accounting practices can still happen, and sometimes banks fail. Sometimes they fail catastrophically, and people can lose money, but it's about as trustworthy as things get.
Tether does not claim to be a fractional-reserve bank. They claim that 100% of the assets are backed by US dollar cash deposits. It's their entire raison d'etre. Except no one is enforcing it. It's not even remotely the same thing as a fractional-reserve bank.