The incoherence of look and feel of modern "applications" is largely induced by commercialization of computer technologies. Each graphical "application" tries to stand out and appeal to the user more than the others (in order to increase the luck of being used/bought), transforming your computing experience into a dollar store.

One can argue that the unification of look and feel can harm uniqueness, but it's irrelevenant nowadays, as almost all computer interfaces are plagued by corporate blandness, rounded corners and hiding actually useful actions under "hamburger" menus. This is the fallacy of inevitable influence of the pop culture that appeared in computing alongside commoditization and popularization of the technology. All of this "safe" rounded corners with all that vivid colours and gradients just screams pop culture, and "more popular" mean more sales means more profit, and all we care is profit. Your computer is now not only a dollar store, but also a candy shop, demanding your attention, filled with ads and popup windows, distracting you from the actual content.

It is better if we yield the uniquiness part to the user, and let the user decide how they see and experience their own computer. Windows XP may be the example of how it should be implemented. Even while the experience part is still incoherent, the looks are, and each "application" of that era looked more or less the same. And the user was in charge which cool "theme" they use and which they don't, according to their own tastes and preferences.

This is example of how to NOT implement user interfaces.
Less actual content, less functionality. The blur, visual effects and vivid colours are more important than the functionality or usefulness of the program. The computer interface should not appeal to the user as much as it can, it just needs to provide the convenient, consistent and efficient way of getting the job done.