You need them to keep money flowing in your system. Like blood vessels. It's a network to keep capital flowing from those with money to loan to those who need to borrow. You don't want to put that responsibility on a single entity, that's a recipe for failure. It's better to spread risk by allowing some to succeed and others fail. There is added complexity and that's a feature, not a bug.

Why don't sales settle instantly? Or perhaps, why can't we do instant money transfers? It's 2021, not 1951.

B/c fraud exists and people expect their money back when it happens. Yes it is 2021, not 1929. Credit and loans are a feature not a bug. If you want to put your life savings into a digital wallet go ahead. There's no protection when someone swipes it.

This is effectively what blockchain is for.