Think of it this way

1. You don’t get sick from the bacteria, you get sick from their excretions
2. Their excretions are on the outer surface (prove this to yourself by letting a piece of meat rot and see that it gets slimy on the outside)
3. Therefore removing the excretions from the outer surface can make a piece of meat safer. Washing seems like it would do that

It can seem very compelling, it can seem like it ought to be true. It used to be commonly recommended.