FWIW, I don't find these gradients especially pleasing. (They're fine, but I find some of them a bit much). I don't know why I would be different, but there it is.

Just throwing a hypothesis out there--I'm not a Mac nor IOS guy, and if those that find them especially pleasing by-and-large are then maybe that's the link. Again, that's just wild speculation without any data to back it up.

Then again, some of these style gradients are used in Ubuntu graphics too (and I again find their presence there a bit much,) so maybe there is a bigger shift towards them.

In any event, I'm guessing it is cultural and not hardwired by monkey-grandpa genes.

At the end of the day our monkey-derived brains still run legacy code wired to find ripe fruits.

I'd guess any gradient is a soothing thing to look at contrasted with sharp edges, which would signify danger or at least potential of physical damage. And they're very natural colors, as are found in sunsets, plants, vistas of the forest - all non-threatening and putting the mind at ease.

Looks like fruit, or water? Or the open sky?

Guess: because they look more natural, as if they interact with the surroundings.

difficult to extract visual features from a flat plane. Gradients give the visual cortex something to process, aiding localization.

Probably the same reason that visual contrast is so important in design.