Why are heating vents placed near windows and doors in my house?

This is typical placement since this is where drafts, either hot or cold, will enter the home. The placement of the vents allows the entering air to be tempered by the heating/cooling system.

Windows are the main source of cold creeping into your house from the outside. The heaters are placed under windows where the coldest air is.

warm air rises, so the chilly air return would desire to be at floor point. once you turn on your AC, then you particularly want vents up extreme to capture the nice and comfortable air. you will desire to open and close vents with the season