Do things exist just because they exist OR they are given that existence?

Question

We know that some phenomena do not exist, like Greek gods, but some exist, like an apple on a desk. I doubt anyone could have proposed an answer to it, but I wonder if there is something which lets something else to exist but not the others, or things exist just because they do so and there is nothing more fundamental than existence to explain it. I even don't know if this bivalence is true to assume things either exist or not, as quantum physics assign probabilities to existence of an electron (exists MORE at some places than the others.)