To answer why we do stretch is to prevent the muscle from shortening and stiffening of the tendons and ligaments are certain. There isn't an exercise that can prevent the aging of connective tissues, so stretching is not a painful process but full of fun with less hazard.
Why should we do stretch?
Ligaments and tendons are made of collagen, which gives them strength, and elastin, which, as its name implies, provides elasticity because the elastin/collagen ratio changes in favour of collagen or scar tissue. If one relies on tissue elasticity for flexibility, then get ready to say goodbye to flexibility. So can't change the mass builders' mechanical properties of muscles, tendons, and ligaments overnight as it will bring more injuries than flexibility. Be mindful that if you stretch to be flexible, you are wrong.
Why one cant stretch or do a side split or dead split?
Listen, there are no short muscles, as our legs boast independent suspension, which means one can split legs without stretching the other. And the cause of stiffness is fear/tension, which tightens our muscles and limits lengthening, known as antagonist passive insufficiency.
Now with today’s lifestyle sitting all day or performing monotonous labour, or exercising incorrectly—one’s nervous system has picked the favourite length for every one of their muscles and prefers to keep it that way. Whenever one reaches too far compared to this standard, the stretch reflex kicks in and reins muscles. An aggressive stretch reflex panics and stiffens up your muscles. So to be factful that it’s the nervous system that brings stiffness.
So the main point is to Master the muscular tension, and anyone can be flexible at any age.
To Illustrate, compare your muscle to a telescopic antenna. Conventional stretching damages the outer tube in a vain attempt to elongate it. In contrast, relax into stretch methods; pull the inner tubes out to the antenna’s full length!