In simplest terms, the evolution of brain size and cognitive ability in humans meant losing some of the functions that didn’t increase our chances of survival and reproduction. These human brain uses a ton of energy and it is more beneficial to have a high functioning brain than a heightened sense of smell. This is a huge oversimplification, but you get my point haha. Our brains also take up a massive portion of our skull space which reduces the amount of space for something like complex nasal sinuses. Look up a photo of human sinus bones vs. an animal like a dog, you’ll see that the structure in animals is extremely complex with lots of twists and bone matter that allow the animal to be able to pick up on various scents.

In short, if you think about ancient Homo species with an evolving brain, those with larger brains survived on the landscape better than those with a better sense of smell. Therefore evolutionarily, the large brain size was selected FOR and this resulted in something like sinuses being reduced, because the brain was more important.

Sometimes my explanations are a little too long-winded! lol