Because the devs never bothered to fix their gaym for modern systems.
And by default you can only play in 4:3 1280x1024.

1. Run the game for once. Choose any resolution in launcher prompt, it doesn't matter.
2. Go to your Documents \ Cold War folder.
3. Copy Config.txt to your game folder.
3.1 How to locate your game folder:
Right click on the game in your library - Properties - Local files - Browse local files
4. Edit the Config.txt via any text editor and change the resolution to which is yours.
5. Download WGSF from their official site.[www.wsgf.org]
Direct download link[www.wsgf.org]
This mod has fixed .exe file that allows the game to launch with proper aspect ratio.
6. Open the archive and open the folder that suits your screen aspect ratio
_FOV version of each folder has adjusted (basically extended) field of view.
7. Replace the original coldwar.exe with the one you chose in the archive.
8. Thumbs up this guide if it helps and enjoy your game!

- Credit goes to WGSF dev(s).