Evolution

Sight, I can understand. The retina responds to a narrow band of EM radiation, which corresponds (give or take) to the highest intensity band generally seen on Earth. Hearing reacts to pressure waves over a large range of frequencies in the air that surrounds us. Touch responds to pressure on your skin from the local deformation. While taste has to monitor a whole host of different chemical patterns, at least it is limited to the small subset of things we stick in our mouths.

But smell - what on Earth is going on there? Your sense of smell is incredible (unless you're anosmic, in which case, my apologies) - it responds to millions (if not more) of entirely unrelated chemicals. If you pay attention, you might notice that people have (to some extent at least) their own individual smell - not their sweat, not their shower gel, but an underlying scent that will be different to another person's. Buildings have their own characteristic smell. Even the slightest hint of an ingredient is often detectable in food or drink - while some French cheeses can be smelled through the box, a plastic bag and halfway across the house. And then you remember that compared with dogs or pigs, we have an incredibly crude sense of smell. I really wonder how the sense of smell developed. Evolution is a madman I tell you.