You trust a decentralized network of computers running code, not just a single one, that's the main point.

There's both centralized and decentralized exchanges available.

A bank is not a single entity, it's made of people converging after reaching consensus

There are a lot of banks, with different interests that usually compete with each other and work under regulations that have been discussed and approved after reaching consensus

There are several different central banks, that work together under regulations discussed and approved because they reached consensus

The "decentralised" exchange already exists