Perhaps you are using hyperbole as opposed to making a generalization. Either way your premise is false and accordingly your conclusion may be drawn in error. Perhaps you’re unfamiliar with recent research, interpreting it poorly or regurgitating someone else’s basis for disregarding scientific research. If you would like assistance Elitefts’ article “A Meathead’s Guide to Reading Scientific Papers” may help you.
If you use a search engine e.g. google scholar, searching for the terms resistance, training and trained, you can see there are many studies on resistance trained populations with more added to this body every day.
While many studies are limited in the manner you described, to discount all studies and their practical and theoretical underpinnings on that basis is the kind of thing that perpetuates the bro science and bullshit that’s plagued the cesspool that is the “fitness” industry allowing snakeoil salesmen thrive, freely exploiting others.