There seems to be no official explanation but per the movie:
TLDR (short) answer:
Walking with shoes is not loud enough for the creatures to hear or distinguish. Especially outside in the nature where there are other sounds present. (depending on where you walk, how you walk, shoe type, how close the creatures are etc.)
Abbott family's perspective:
- They didn't know the outside world too much, if anyone survived or how they survived.
- They didn't know how sensitive the creatures' hearing is.
- They chose to be as quiet as possible.
- They had to walk through different places/terrains outside and within the property.
- They decided to use sand paths (it would make more noise if you walk with shoes on sand).
- Shoes can still make loud/distinct noises depending on other factors. (rubber-soled shoe squeak, wet shoes etc.).
- He knows outside world more than Abbott family. (per his dialogue and experience, for example, he knows what other survivors have become).
- He finds out that the sound of walking with shoes doesn't attract the creatures.
- Presumably, he wears a type of boots that would make less noise. Also, if you check the walking scenes, he is walking slowly and carefully.
- He is mostly spending his time in the foundry and the bunker under it. There is 3 feet of concrete above the bunker. He is less worried about making dull sounds. They can even talk in the bunker and he says that "That must be right above us to hear".
- As the nature has lot of different sounds, the sound of walking with shoes just blends in (if you are careful enough). The only exception to this was the final scene where they walk inside the radio station. He walks extra slowly and carefully there.
Note: To be more clear, I want to add more details about the universe and the hearing of the creatures. When the alien creatures first arrived at Earth, they were attacking every sound source and its vicinity (they attacked towns/cities quickly as there was more noise). As time passed, Earth became quite when the civilization has collapsed and few survivors left.
By now, we can say that the creatures have accustomed to the usual sounds of the nature. Possibly, they attack other animals that make distinguishable sound. When they are not close enough, they choose to attack loud and/or distinct sound sources. When they are close, they make extra effort to hear the faintest sounds by opening and closing their vulnerable ears and they also use echolocation with the their clicking sound. In this case, it becomes almost impossible to escape them unless they are distracted by another sound. The characters could survive the close encounters in some occasions when the creatures were distracted by other sound sources like the water flow.
As I mentioned before, walking with shoes slowly and carefully would not make a loud/distinct sound for the creatures to notice, unless they are very close and making extra effort to find you. Possibly, you might need to be extra careful at night time as it is more quiet and there are more sounds in daytime in the nature. Emmett possibly avoided walking at night. In one of the scenes, they walked at night to arrive to the dock but again he was walking carefully.
Other than that, other animals walk or move and they would even make more noise than a careful human. The alien creatures walk also. We find out that boat people are wearing shoes also, as seen in the dock scene.