Think of your muscles as your body covered in skinny jeans you cant take off. You know how uncomfortable it is to put your jeans back on after they have been washed in hot water? That is what “not stretching” your muscles is exactly like. The long muscles pull and scrunch and so instead of having “space” where the bones of joints meet, the bending bones are pulled too tight together and start to rub and create friction.
Stretching your muscles is similar to breaking in your skinny jeans.