There are elements which can be tested. Measuring brain activity etc...
I suspect science/scientists are largely embarrassed by proposing religious experiences as a valid area of research. Here an interesting paper I've come across.
http://userwww.service.emory.edu/~philrnm/publications/article_pdfs/JCC%20Intro%206-25-04.pdf
Which might mean the lack of contemporary evidence being cause by a lack of motivation.
The tools may still need to be developed to enable the accurate measurement the traverse. That doesn't mean the traverse is not there nor that it's not being crossed by people.
So we have testimony of individuals who claim they've crossed this traverse. We end up relying on their descriptions, which is not so reliable. We've no means to measure this traverse so we can't determine objectively the accuracy of their testimony. They still have the experience of crossing. That's what they have to rely on until, if science develops a method to measure, model and test exactly what that experience is.
People got sick even before science developed the means to test for pathogens and vectors of disease. Would you expect these people to have accepted their sickness was all in their mind because the science was lacking?