TLDR of the rest of the post, the unknown state of the particles is not the same as flipping a coin and refusing to check the result. There is not an actual state the particles are in until we learn it. Which is why quantum mechanics are so insanely counter intuitive.

You'll not be able to get a satisfying ELI5 answer to this. Quantum mechanics are extremely counter-intuitive. You may think, like I did, that the unknown state of something is actually sort of already set in stone but we just don't know it yet. I thought this because scientists are very clear about not saying things they haven't verified so of course they couldn't know the state of something until they measure it. You may think it is like flipping a coin and covering the result. You don't know the result but it is already one way or the other. That's a typical ELI5 answer to this but it is incredibly misleading because quantum states are not so-called "classical hidden states" which is like a fancy way of saying "I flipped a coin but haven't looked at the result yet."

I find this section on Wikipedia useful for seeing how Scientists know that quantum states are not the "classical hidden states" I naively suspected. https://en.wikipedia.org/wiki/Double-slit_experiment#Mach-Zehnder_interferometer