Answer:
We are well-built for hamstring stretching. We have good leverage for it. Thanks to the arrangement of our parts, there is almost no limit to the amount of tensile force we can apply to the hamstrings — much more than the muscles can actually tolerate. If you could stand the pain, you could tear your hamstring muscles. You could literally rip them apart. Wow.
There are a few other muscles like this in the body.
But the opposite is true of several other muscle groups in the body. Just as anatomy facilitates strong tension on the hamstrings with convenient and powerful leverage — applicable simply by leaning forward — many muscles just do not allow full elongation and/or conveniently applicable and powerful leverage. There are several muscles that you cannot barely stretch at all, let alone tear, no matter how hard you try.