Dense UIs certainly have a place. But, simple UIs are not a fad, as it seems some people in this thread see it. The goal is as simple as possible, but no simpler.
The majority of applications and websites you interact with should be simple, and a few should be complex and dense. The reason is that you aren't an expert at most applications and websites, and you want them to be simple, so you can do the thing you want to do without investing much effort. But for applications you know really well, and use all the time, you want them to be more dense, so you can get more things done with fewer steps.
Because there is no easy, cost-effective, or even feasible way to scale the same application's UI complexity smoothly from newbie to expert, the designer almost always has to try to thread a path between the two extremes. This path has to make sense for the use cases they know about, and the largest share of the users they want to serve. This is extremely hard, not extremely simple, as it may seem from an observer's position.
I think you are correlating "simple" and "sparse". Sparse looks simple but in practice might be harder to use. Mobile restaurant menus, as discussed here a lot, are an example where this doesn't align. In general having to scroll a lot when I could see all information needed at once makes things harder, not simpler.
The majority of applications and websites you interact with should be simple, and a few should be complex and dense. The reason is that you aren't an expert at most applications and websites, and you want them to be simple, so you can do the thing you want to do without investing much effort. But for applications you know really well, and use all the time, you want them to be more dense, so you can get more things done with fewer steps.
Because there is no easy, cost-effective, or even feasible way to scale the same application's UI complexity smoothly from newbie to expert, the designer almost always has to try to thread a path between the two extremes. This path has to make sense for the use cases they know about, and the largest share of the users they want to serve. This is extremely hard, not extremely simple, as it may seem from an observer's position.