There are so many different aspect ratios, and so many devices with said aspect ratios, it would be a bear impossible task to compile data on every device on the market, off the market, and future market, so it's better just to support the most common ratio and adjust to suit all (though it will look slightly different on each device that's not your set aspect ratio).
This will only really be the amount of background that can be seen. UI elements can remain fixed in their respective positions.
I get what your saying but there's no need to offer different resolutions as a setting within your game. Just stretch your background around 20% larger than your viewport and use scale inner/outer (depending on your game, UI etc) and you'll be set for multiple ratios.
And as I said, you can always use Scale Layout if you want to give the appearance of being zoomed in or out within your game
Every single game out there supports different resolutions in the settings and you're saying I don't need it? I'm sorry mate but it makes no sense.
And of course, because of that I'll offer the option for scale outer as the only option to not get a terrible experience in which, in many cases, will be half of screen with black bars around. I'm using pixel art, so the ideal would to offer a letterbox integer scale full screen mode to the most commons resolutions so I could avoid the pixel distortion for most cases and then the scale outer mode would be a secondary alternative, not the only one.
Again, I don't see why this is not an option in C3, 
ASHLEY. Specially for pixel art game.
Btw, my project and my monitor are 16:9 and I still get black bars.