But he did. He called it Gravity.
He didn't let figuring out "how gravity works" hold him back. He just figured out the equations that predict its effects based on observation. The unknowns happened to conveniently coalesce around a single constant G, but that doesn't make them "known".
[IANAP - do we have any convincing popularly accessible models for how gravity works even today?]
Anyway, that's kind of similar to going "oh I don't understand the subatomic mechanism that is causing this interference pattern, but the pattern itself is consistent so lets call that P!".
Unless I completely sped past the point you were trying to make - which is completely possible at this subatomic level of reasoning (-:
Newton was looking for methods to manipulate (push, pull) objects at a distance without touching them. He had strong ideas about the occult and what we today call black magic, in addition to being an alchemist. He devised that the Earth was manipulating the untouched Moon (capitalized, proper noun) and sought to explain the influence of the Earth on the Moon.
Many science-related articles on Wikipedia suffer from good-meaning but over-zealous gatekeeping of religious influences on science. So I'll just recommend to you to read a biography or three on Newton. His entire story is fascinating and we can see how tiny events in his childhood (death of father, influence of priest uncle) have affected the history of physics.
[1] https://en.wikipedia.org/wiki/Isaac_Newton [2] https://en.wikipedia.org/wiki/History_of_gravitational_theor... [3] https://en.wikipedia.org/wiki/Newton%27s_law_of_universal_gr...
QM effectively seems to be non-causal - or at least, much less causal.
Tidying this away under the heading of "It's probabilistic, let's just do the math and not worry about why" seems to be ignoring some very basic questions.
I can't seriously imagine Newton saying "Well, the Moon stays in the sky instead of falling down - and that's frankly a bit weird, but I'm not going to wonder why because it's just a mystery and anyway, I can predict its orbit with these epicycles."