Same as for desktop .. every Application in Windows (say) has a different UI; not just emus or games, but business applications. Heck, look at Microsoft, mxing it up with the ribbon which quite a few peopel absolutely hate, or the major changes going on in Linux land (Unity, KDE4, Gnome 3 ..) -- it is not limited to justy our little platform. You can't really lock developers down, they'll do whatever is easy, or whatever they think is best for their niche, or just whatever works
There have been many discussions about trying to standardize it, but even qaside form getting devs to rework old code, to advertising the effort to new devs, there is the big problem that not all emus are alike; straight old consoles liek the Atari 2600 have limited options, but some consoles had options (more RAM..) or addons like the genesis 32x with or jaguar with the toilet seat add-on; plus some amchines liek Atari ST or Amiga have floppies, carts, RAM or interface changes (joysticks, mouse, hard drives..) and differernt chipsets based o machine model; Sometimes its hard to come up with a satandard UI.. some machi8ens have full keyboards, some just a single analog joystick (Vectrex).
Its not as easy as you think
Another effort is ongoign right now, in just having devs write whatever UIs they want, but maybe using standardized nomenclature and help artwork -- to help make it clear what buttons do what. But even that is a hard battle. Heck, we've been going over this for 15 or more years, on every new mobile device; people wanted standard UIs for Palm and Pocket PC, for emus on their GBA and DS, on their PSP, on their gp2x and so on
jeff