Because they pick up dirt from the environment and skin and other stuff from us. All of this plus our body heat make it it a nice place for bacteria or fungi to live and grow. Washing kills the bugs so that we smell nice and our clothes last longer.
Is this something you often ask your parents?
You don’t have to wash your clothing, it’s just more hygenic for the reasons the other scientists have listed.
There is a school of thought though that suggests that there has been a massive rise in allergies and asthma over the decades because we have become too clean -it’s called the hygiene hypothesis. Essentially, we need a bit of muck to keep our immune systems busy otherwise it might get bored and causing unnecessary diseases.
But I have a question for you which I have always wondered about….Why do we need to iron?
We need to wash our clothes because we sweat/excrete stuff from our body, as well as picking up dirt and contaminants from our environment. If we did not wash our clothes we would be at risk of harbouring bacteria and increasing the likelihood of infections/disease.
Plus it means our clothes look and smell nice and last longer.