Einstein, as everyone knows, was one of the most intelligent and insightful theoretical scientists ever born. His work on the photoelectric effect, which posited that the frequencies of light emitted by a material are quantized, rather than continuous, is what won him the Nobel prize. His thinking influenced the great thinkers behind the formulation of quantum physics as we understand it today: Bohr, Dirac, Heisenberg, Schrödinger, Planck, etc. But nobody, not even Einstein, has been able to come up with a way to describe the physical basis from which the laws of quantum mechanics (which we know to be true based on many, many years of experiments which have failed to falsify them) can be arrived at.
Contrast this to a topic for which Einstein is better known, relativity. The mathematical framework describing the effects of special relativity had already been partially discovered by Lorentz, but we credit Einstein for its discovery. Why? Because he postulated two simple rules from which these laws can be derived: 1) The speed of light is the same in all inertial reference frames, and 2) the laws of physics should be the same in all inertial reference frames. How about general relativity? A similar idea, that a person inside of an elevator should not be able to tell the difference between sitting stationary on Earth and accelerating through space at a rate of 9.8 m/s^2.
Einstein had opinions on the physical basis for the laws of quantum mechanics. But they were not necessarily the most popular opinion at the time, and definitely not today. As tylerhou posted in another comment, tests of Bell's theorem more or less disprove the existence of local hidden variables, meaning local quantities that behave deterministically but not measurable. It is still possible that there are nonlocal hidden variables, but this is not a popular opinion these days for a variety of reasons.
I would also like to point out that the various "interpretations" of quantum mechanics (e.g. the Copenhagen interpretation, pilot wave theory, etc) frequently fail to address what the nature of physical reality, i.e. what is the starting point, what is the axiom, what is "real". Are wave functions real? Do they actually exist, or are they merely convenient mathematical tools that can be used to describe reality? Are pilot waves real? Are density matrices real? Are Greens functions real? Are quasiparticles real? (n.b., most of these things are described as complex-valued quantities ;) )
Contrast this to a topic for which Einstein is better known, relativity. The mathematical framework describing the effects of special relativity had already been partially discovered by Lorentz, but we credit Einstein for its discovery. Why? Because he postulated two simple rules from which these laws can be derived: 1) The speed of light is the same in all inertial reference frames, and 2) the laws of physics should be the same in all inertial reference frames. How about general relativity? A similar idea, that a person inside of an elevator should not be able to tell the difference between sitting stationary on Earth and accelerating through space at a rate of 9.8 m/s^2.
Einstein had opinions on the physical basis for the laws of quantum mechanics. But they were not necessarily the most popular opinion at the time, and definitely not today. As tylerhou posted in another comment, tests of Bell's theorem more or less disprove the existence of local hidden variables, meaning local quantities that behave deterministically but not measurable. It is still possible that there are nonlocal hidden variables, but this is not a popular opinion these days for a variety of reasons.
I would also like to point out that the various "interpretations" of quantum mechanics (e.g. the Copenhagen interpretation, pilot wave theory, etc) frequently fail to address what the nature of physical reality, i.e. what is the starting point, what is the axiom, what is "real". Are wave functions real? Do they actually exist, or are they merely convenient mathematical tools that can be used to describe reality? Are pilot waves real? Are density matrices real? Are Greens functions real? Are quasiparticles real? (n.b., most of these things are described as complex-valued quantities ;) )