Bacteria aren’t smart. They don’t think “let’s go over there to eat”. They go from high pressure to low pressure (lots of bacteria to fewer bacteria).

Basically If you leave the wound open the bacteria can’t burrow into healthy tissue. They are met by a host of immune cells and connective tissue that makes the process hard. If you close the wound you trap the bacteria in a warm, wet space with plenty of food. They will multiply and pressure will build up and start separating the tissues and the bacteria will move into that new area. That’s an infection. If they have somewhere else to go that is no longer an infection. Leave the wound open and it will heal towards to skin, then the skin will heal over the healthy tissue and there will be no pocket for bacteria to hide in. This is a VERY simplistic way of describing the process.