I think it comes from a lot of things. I mean, it's certainly at least in part due to the ease of access. I think for some people it can create a sensation of "accomplishing more". Why limit your social interaction to only those physically around you, when you can be social-multitasking?
That being said, I know that the simple act of just "doing stuff on a phone" has become an extremely common form of compulsive behavior. People (and I include myself in this) will take out their phone and start doing shit without even realizing it, and without actually having a specific reason for initially doing so. It's like someone checking their watch multiple times in a short period, even though they probably don't really need the time. The difference is that with a watch, there's not exactly a whole lot to engage in. You can't justifiably pretend to stare at your watch for more than like, five seconds at a time before it starts to seem weird. With a modern smart phone, that's not the case, and in fact it's pretty much designed to keep your attention. Especially once internet became common and easy to use on mobiles. So it's the same sort of compulsive behavior I think, except now instead of a watch (or whatever), we're just casually checking a device that literally has near instant-access to the majority of human knowledge -- aside from being most people's primary link to their social life on one level or another.
Beyond that, there's also the fact that checking your phone is sort of like temporarily partitioning yourself off from a social situation. So people will do it when they're uncomfortable, or if they don't know much about the current topic of discussion, or if they just don't care about it, so on and so forth. Thus, where in the past we would basically just have to wait those moments out (and thus be more encouraged and more likely to try and find some kind of entrance into the conversatrion), now we just...check our phone. Combine that with aforementioned attention-swallowing nature inherent to a smartphone, and it becomes both more common and more noticeable.
Will this change? I doubt it, at least in the sense that I don't think we're going to regress. I do think that when (and if, though it seems inevitable) smartphones become less conspicuous and even easier to access, like say through Augmented Reality for example, then it will cease to be the same type of social shield it is now. But that's a ways off and is obviously super speculative.