Welllllll, there are many different explanations on that thought.
First, there are many things, in any field of Medical, Engineering, etc., where something works really well on paper, or in theory, and is an absolute failure in real life. In field medicine, we teach medics first in the classroom. THEN we go out and teach them in the field. Some of the things that we teach them in the "BOOK" is contradicted by what is taught in the field. AND there are many tricks instructors have learned by trial and error (or desperation), that we show them in the field, which aren't in the "BOOK". It's an old instructor's trick to make us look smarter.
Another issue is that, what works in theory, (talking about the medical field ONLY now) people have $$$$$$ invested into, and don't want to admit that it doesn't work, soooo they keep re-working and re-designing it. That is the case of the THUMPER. The first incarnation of the THUMPER was also in the late 70's, early 80's, along with standardized CPR. The THUMPER was an electrically powered, pneumatic (I know, sounds like a contradiction) device, which was mounted to the stretcher, directly over the patient's chest. It is intended to replicate a person doing CPR Compression's. Buuuuuttttt, with each patient being of different sizes, if the THUMPER was not set up EXACTLY RIGHT, it would wind up being set too shallow, and not doing any good, or too deep, and turning most of the chest cavity including the heart into jelly.
Every few years, some company comes out with a new version of said device, claiming they've fixed all the issues the previous types had, and that THEIRS will actually work. Some work better than others, but so far, none have ever delivered on all their promises.