Because in a hot day, there are a lot of heat coming from the sun. The heat will reflect off from the road. When there are a lot of it, you can see it.
When you see far, you in fact see MORE things than seeing near. For instance, the HEAT in the airport and the road nearby.
In a hot day, the heat rises into the air. Due to the hot temperature, you will see 'steam' rises into the air especially in the airport, in a runway where there are nothing to shelter, nothing to cover up the heat. The road absorbs all the heat and give up as what you say 'vapor'. You will see also in hot climate places like Texas, Arizona and New Mexico`s streets.
Something close to you, like the place where you are standing, you cannot see the 'steam' rising up. It is just too close. You are actually seeing the vapor directly (in your terms) being sucked into the air. You are ON the vapor. That is why.
