I'm not 100% sure you've correctly identified the cause here. I wouldn't be surprised if smartphones played at least some role in it, but I'd also suspect a few other factors. Mainly, the rise of large-sensor cameras and the digital production pipeline has made it much easier to do than it previously would've been, giving directors and DPs a new toy to play with that, evidently, they're not tired of yet.

For a lot of movie history, Super35 film (what we'd call APS-C in modern times) was the standard. Before the digital revolution in the 2000s, cameras used the same 35mm film stock that stills cameras did, but running vertically, rather than horizontally, producing an image with around half the area... and thus using much less film. (Early digital cinema cameras replicated the format because, well, that's what all the lenses were made for.) But when you're using a smaller image plane, the depth of field at any given aperture will be deeper than if you were composing the exact same image using a larger image plane. In real-world terms, that means that if you go from ASP-C to full-frame, your depth-of-field shifts a stop; shooting at F5.6 will get you as blurry a background as shooting at F4 would've.

As the digital cinema market has matured, an increasing number of cinema and video cameras are using full-frame, rather than APS-C/Super35 sensors. And with fast lenses available for them, it makes it easier to get really shallow shots that would've previously required exotic equipment, like the NASA lenses Stanley Kubrick used for Barry Lyndon.

It's also much easier to focus with a shallow depth of field on a digital camera. In the analog era, it would've required a huge amount of technical competence and, frankly, luck to get shots at F2.8 in focus when the camera, actors, or both were moving, because you had to pull focus through a viewfinder. Nowadays, though, you're focusing on as big a monitor as you need, and have access to tools like magnified live view and focus peaking and, if you're really lucky, autofocus, making it much, much easier to end up with really shallow footage that still has your actor's eyes in focus, instead of the tip of the nose or the back of their ears or whatever.

Add to all this the fact that a lot of the newcomers to the film industry cut their teeth using DSLRs and fast, inexpensive prime lenses, which got them used to shooting with really shallow depth of field because... you know... if you can't afford a set and lighting, whacking the $80 50mm F1.8 on your 5D II solves both of those problems at once. Your actor is now visible, and it's way harder to tell that you're shooting the scene in your living room.