Why do people call underwear shorts?

Many people call underwear shorts, that's what we call them. Actually there is a difference between underwear and shorts. Underwear, generally refers to underwear, under normal circumstances, people are afraid to wear underwear outdoors; Shorts vary in length, from the length to the calf, and the short ones are a little longer than underwear. Under normal circumstances, underwear should be worn in shorts, and it can also appear outdoors. And underwear can't be worn on shorts. So underwear is not short, but it is not shorts.