why should you not stretch the quads ?
A couple of reasons.
First of all, you want your quads to be tight to support the knee, you don't want your quads & calves/hams pulling the knee from both directions, the quads must dominate.
Second is that the most common stretches for the quads are also stretching the ligaments in the knee. Ligaments are a bit like leather - once you stretch them, they don't go back.
Finally, it is rare indeed that your quads get so tight that it would cause problems.
I trained with a guy who was in the 4 x 100 metre relay in the Atlanta Olympics - he helped me get back into shape after a knee op. A lot of the stuff we did was power exercises - but he put me onto why runners get knee pain and explained why most people don't need to stretch their quads. He's also a qualified doctor.
You have to do weights too - he's the guy that told me running does not give your muscles the strength they need to support your running.
Since following his advice, I have had knee pain only on the occasions I didn't bother keeping my hams & calves stretched. Stretching is, after all, boring as batshit.