Other people are giving you the right answer - it's about surface area and the gills collapsing. But I feel like that needs a more ELI5 description:

Picture someone with long straight hair going underwater. In water, a fish's gills are like a person's hair behaves under water. It's all spread out in the water and every individual strand is floating freely. There's a lot of hair surface touching the water.

When a fish leaves the water into air, their gills act like wet hair when you get out of the water. It all "collapses" into clumps. The amount of contact between hair strands and the surroundings is a tiny % of what it is under water.

Fish rely on their gills being all spread out and free-floating to have enough surface area to pick up enough oxygen. It's not that gills are unable to extract oxygen from air and can only get it from water. The problem is their physical structure prevents them from touching enough air to keep the fish alive.