Agreed, from what I’ve seen it’s less about the muscle or tendons being “short” as it is your brain being protective and preventing lengthening to stop you from hurting yourself. So stretching is less about making things longer as it is about teaching your brain that you aren’t going to hurt yourself in different positions so that it lets you get there. I’ve read that not matter how inflexible you might be, if you are knocked out or given anesthesia then someone can basically put your body into positions that would make cirque du soleil performers jealous.
One of the best things that I have found for increasing range of motion is longer (30+ second) stretches with deep breathing. On each exhale you try to fall a little bit deeper into the stretch.
EDIT: It is as much about relaxing as anything. A stretching session might be lightly physically strenuous but mentally it should be as relaxing as possible. It may be different for you and may take some time to get here but about 15-30 seconds into a stretch I usually feel my muscles “give in” to the stretch and sort of “fall” into it. But you need to be relaxed and go slowly so your brain’s protective mechanisms don’t kick in.