Because there is not absolute proof. Nor can any man be he scientist or just a layperson provide any that can be conclusive beyond doubt.
Science don't deal in "absolute proof" but it does deal with empirical evidences.
(A) The more evidences that are available and verifiable, that support the statement (statement as in explanation and the predictions to go with the explanation, e.g. found in theory & hypothesis), the more probable is that statement.
(B) If more evidences goes against the statement than support the statement, then the statement is less probable, therefore it shouldn't be accepted. Such statements should be discarded as debunked hypotheses or refuted theories.
(C) And if there are no evidences whatsoever, to support or refute the statements, then those theories or hypotheses should be treated like the (B) - "refuted" and "discarded".
(D) And those statements that are "falsifiable" and "testable", should be considered as debunked, and treated like (C), as not probable but also it should be branded as pseudoscience.
So sum these 4 scenarios up:
- Statements (explanations and predictions in hypotheses or theories) are only accepted in science if the evidences showed that they are "probable".
- If the statements are "untestable" (like D), or have zero evidence (like C) or the evidences show that it is "improbable" (like B), then the statements should be considered improbable, refuted and discarded.
You don't keep bringing up the same refuted hypotheses or refuted theories; you would throw them away, and start something new and fresh.
Members of the Discovery Institute (DI) and Intelligent Design (ID) advocates keep bringing Michael Behe's refuted papers on Irreducible Complexity (IC), on the ground that it is unfalsifiable and untestable, and considered nothing more than pseudoscience...and yet they keep bringing to life, only demonstrated the ID adherents are unwilling to accept logic and reality.
The Discovery Institute is not scientific organisation and Intelligent Design and Irreducible Complexity are not science, so they shouldn't be treated as such.
Creationism, especially the Young Earth Creationism (YEC) and those who accept literal interpretations of Genesis creation and flood, would fall under D category, like Intelligent Design, as pseudoscience, because YEC is not falsifiable and not testable.
For example, if God as the Creator, is responsible and cause for creation of the natural world, then there should empirical evidences for the "cause" (hence for God). But God cannot be tested and it is not possible to find actual evidences for God's existence. That mean the belief in god is based on blind faith, not on evidences.
Science relied on probability and empirical (and verifiable) evidences, not on "absolute proof". Evidences are used to either refute or verify any statement, so the evidences must be testable.
This "absolute proof" fall in the realm of mathematics.
Proofs are mathematical statements, like mathematical equations and mathematical models. It is not real-world solution.
Although science can contain some maths, (proofs like equations and formulas), that's not what make theory being accepted as science. What make theories being "scientific" are the evidences that can be verified.
Evolution is accepted, because the number of evidences that support various mechanisms for biological changes (e.g. Mutation, Genetic Drift, Gene Flow and Natural Selection).
Evidences are not just found in fossils, but also from observation of the current and extant species of plants and animals, and on organisms such as bacteria and viruses.
We know that Germ Theory is factual, and understanding GT relied on understanding evolution.
Natural Selection not only occurred among species of animals, but it also in plant world.
Plants have been observed to change, by adapting to their environment. Some species of plants within the same genus, have changed, because of different soils, for instance, the species can changed due to the acidity of the soil, or there been too much alkaline, or the soil is too dry or too wet. New plants, as offspring, will be different, if they are moved to different locations.
