Because you won’t stop stretching them. Many people who have consistent hip flexor tightness would be a lot better off if they just stopped stretching them. This often provides only a temporary relief, giving just a small window of comfort. And guess what? The more you stretch them, the shorter that window of relief becomes, until you’re at the point where you’re stretching them multiple times a day for a long duration just to feel good! That’s no way to live!
Another way to look at it, if you stretched a short and tight muscle and it regained length, it shouldn’t get tight again, should it? Whereas if the muscle wasn’t technically “tight” but rather holding excessive tone in order to keep your spine from looking more like a losing game of Jenga, stretching it will just give more opportunity for low back pain, and quickly lead to the muscle tensing up again to defend the spine. This is the problem with conventional hip flexor stretches.
Tight hip flexors occur for a variety of reasons. Those who run frequently or engage in other activities that put strain on the hip flexors are likely to experience hip flexor tightness at one time or another. A blow to the hip or poor conditioning can also be causes of tight hip flexors. These causes can usually be attributed to tiny tears that occur to our hip flexors through rigorous activity.