I'm a graduate student in the biomedical sciences. I perform scientific studies and occasionally publish in peer reviewed journals. I am not a clinician, and I do not run clinical trials.
Clinical trials are not basic research, though they rely on a great body of basic research, and are themselves experiments. Requiring that clinical studies have clearly defined end points serves the purpose of ensuring that the results are robust and well understood. This is important because the end goal of a clinical trial is developing a treatment of some sort that will go into many humans, and mistakes can be very harmful.
Choosing an appropriate endpoint for a clinical trial can also be very challenging. Say for example you're trying to advance a drug candidate for treating cardiovascular disease. You have many choices for how to measure that - chest pain, resting heartrate, cholesterol etc. It's very much possible that your drug does improve cardiovascular health, but your trial can fail because you chose the wrong endpoint.
When I perform an experiment, I do so with a hypothesis which is sometimes proven right, and sometimes proven wrong. Either way, the results can be interesting and form pieces of the large puzzle that is a scientific study. Further, questions that are addressed in a scientific study are generally going to be more broad than those addressed in a clinical trial - we don't really do exploratory clinical trials, but exploratory research is a very important part of science. For these sorts of studies, it is often difficult or impossible to have a well defined endpoint such as is required in a clinical trial. One example for you: I hypothesize that enzyme X has function Y. During my studies, I discover that enzyme X actually has function W! If the evidence I present for enzyme X having function W is solid, my results can still be great and useful, even if my initial hypothesis was wrong.