@iamanengineertoo
I don't quite understand why flat earthers have this notion that if one can't observe the actual cause of an effect, then the cause doesn't exist.
I'm no zeteticist/zetetic, but I think you may have misunderstood the premise a bit.
My, limited, understanding is that you objectively/unbiasedly collect observation/evidence and conclude/deduce/infer directly from that. You do not speculate a cause, but rather look for correlation/causation in the data and allow it to lead you to a conclusion.
In general though, yes - in all empirical science of any kind - if you can't observe/measure something then it is not a part of science. This applies to speculative "causes" and everything else. If it can't be measured/observed then it can't be (empirical) science.
For example, we know that germs are real because of the effect that various types of germs have on the human body and other bodies.
We know that germs are real because of the ability to see/observe/measure them with the advent of the microscope. We (pasteur) developed germ theory, in part, due to observing the effects they appeared to cause (of course, along with confirming and/or introducing their presence). Only AFTER knowing/measuring/observing that germs exist, could anyone speculate that they may be a cause of this or that - scientifically speaking.
Even today germ theory suffers from all the same problems it did originally. For instance, we have staph and strepp all over our skin. We have tetanus in our saliva. The problem and cause of the sickness is obviously NOT the presence of the bacteria / virus / germ. It is primarily to do with biological balance and immuno health.
Why do FErs sometimes claim that the earth is flat because they feel like they aren't moving, it appears flat, etc.
In the context of the zetetic flat earth society (not flat earth research at large), it is because those first hand observations/measurements are the basis for further deduction about the world. It works that way in "standard" science too! They DO tend to shun "i felt it" or "i saw it" as scientific "data", but the logic is identical. The world does appear flat, and we do not feel any motion at rest.