Why do games only allow certain resolutions? Only a few games have resizable windows when running in full screen mode. All others have a finite fixed set of allowed resolutions. But why? I see some reasons, but none of them seems to be that important to me that would justify such a user-unfriendly feature.

Example

Let's use Dark Souls II, because it's the first I can think of. The allowed resolutions are:

800x450
1024x576
1152x648
1280x720
1440x810
1600x900
1680x945
1920x1080


What is the reason to not allow the resolutions 800x451, 800x452, ..., 801x450, 801x451, ...? What it I think can't be:


The UI. If we keep minimum and maximum window sizes as limitations, the only issue I can see here is with settings with a weird aspect ratio (1920x450, 800x1080). Otherwise, there shouldn't be any problems with free scaling between the resolutions.
Camera, Viewport. Same as above.
Note that in your Dark Souls example, all the resolutions listed are the exact same aspect ratio.  So the question is, why doesn't it support arbitrary aspect ratios?  

The answer is that it costs time and effort to support arbitrary aspect ratios, and if that money spent doesn't translate to increased profit, then it doesn't make sense to do.


This answer covers the 2d layout issues, which includes extra time and effort to create a versatile layout that can be resized so that elements do not overlap, stretch, or pixelate in an undesired manner, and that the overal UI maintains the best user experience.
The developer would have to ensure that the window doesn't get too large or too small.  Does it make the game any better if the user can drag the window size to be smaller than 800 x 450?  probably not.
you have to define the behavior of the camera when the aspect ratio changes. 


For example, take the following scene:



If we allow the user to change the horizontal size, we can decide to "zoom out" horizontally so that the same area still shows up horzontally but now we can see more vertically.



Or, instead, we can keep the same zoom level but just clip the viewing area more:



Note, that if you don't specifically handle the changing aspect ratio, the default graphics pipeline behavior will be to stretch the image:



These decisions take time and effort to design, code and test, but management has to decide if it actually makes the user experience better enough to be worth the time.  Apparently, most games have decided its not worth the effort.  I'd even go further and suggest that allowing users to screw up the best layout of the game is a bad feature.