Gardening nerd take: Fruit trees are constantly being pollinated by whatever random cultivar is in the next orchard over or whatever wild version of the tree they were domesticated from just happens to be growing in the woods nearby. The hard part isn't getting a variety of different kinds of apples, it's getting the same kind of apple consistently. What growers really want is an apple that's tasty, has a nice texture, keeps for a long time in storage, and isn't too fragile to transport to stores far away from the orchard.
This is why most fruit trees, along with loads of other kinds of ornamental and edible plants, aren't grown from seed. The rootstock (a hardy variety of your tree that may have some bonus qualities like disease resistance) is grafted onto the scion (a cutting of a tree that produces a known cultivar of the fruit you're after). So the apples you buy at the store come from clones of a really good, or at least really reliable and profitable, apple tree.
You can actually get funky fresh with it if you want and graft different trees in the same family together, so you can have an apple where every branch bears a different variety or a fruit salad tree where every branch is a different kind of stone fruit. You can also crossbreed certain fruits to create hybrids!
Americans happen to be able to see more variety with our apples because we produce them on such a massive scale that we've performed feats of engineering like building huge oxygen-controlled warehouses to store them for longer. Europeans actually breed a lot of super cool fruit varieties too, but like us, most of your available fruit in the big grocery chains is probably whatever grows biggest and keeps longest.