The problem with Why

I've heard it said that "Why?" is not a question for science, but a question for philosophy. Science is only about the "What". It describes the behavior of things, not the reasons for it.

That's simply not true. Science answers every "Why" question that it can competently answer, and it would answer more questions if we had more knowledge to be able to.

The issue comes up a lot with quantum mechanics, such as "Why is a photon's behavior probabilistic for example in the double-slit experiment?"

It is however not necessarily a problem of "not enough knowledge".

I think that one of the main problems with "Why?" is that the asker is not just looking for a cause or a description of a mechanism as an answer to the question, but instead is unintentionally asking "What explanation is there that can be described in terms of things that I've experienced?" The asker is looking for a "common sense" answer, and there is no reason why every physical phenomenon should have an analogue in human experience. Thus, it may be that a satisfactory answer for something like "Why do things behave probabilistically?" might not exist in common-sense English, using words that describe things that we experience directly.

The answer to every "Why?" might be "That's the way it is." It might not be possible to always break down the answer into simple-to-visualize concepts.

Addendum: To paraphrase Richard Feynman, "why?" is not a good question to ask, because it can be asked again of any answer given, until eventually there is no possible answer. However, I feel there is no problem in answering every "why?" that can be answered, so long as the asker understands that not all of them can be.