A bunch of great points have been brought up here.
The scientific method is one of the most important developments in all of human history. I can think of very few things that have been as instrumental in the advancement of mankind.
That said, it’s still not perfect. Research, particularly the type that holds up to rigorous scrutiny, is very expensive. That money comes with strings. In order for small studies to get the funding they need to become large studies, the results oftentimes need to reflect a predetermined outcome that benefits the grant writer, or be inherently profitable. History is full of examples of the scientific community at Large ostracizing someone for an unpopular idea that later turned out to be accurate.
That’s all a bit off topic though. To the OP’s question, there hasn’t been a great deal of scientific study for most PE and will probably not be for the foreseeable future because it is so difficult to monetize. There have been some that show positive and reliable results with extenders. If it was only the stretching of the suspensory ligament, anyone claiming girth increases would have to be lying which I don’t accept. I’ve seen increases in girth myself which means there are changes happening in the penile tissues themselves.
Starting Stats Jan 2019 - BPEL: 6.75", MSEG: 5.875"
Now - BPEL: 7.0625", MSEG: 6.25"