Well, waterproof panniers are considerably heavier and more expensive. Covers weigh next to nothing, I don't think they are an issue unless the rider also counts the toothbrush bristles.
Some people simply do not ride in the rain, do not need waterproof gear and feel perfectly fine when they return from their tours with non-waterproof panniers. There isn't much to understand or not understand.

On a related note: Some people wonder why on Earth I don't tour in cycling shoes with stiff soles. Some people wonder why on Earth I don't tour in sandals. It's a conundrum, this world, really.