Because ram is incredibly cheap these days for how much you get. Games only generally use a certain amount of ram hence developers are able to specify minimum requirements.
Games run "live" in a way where they are loaded into memory and use the fastest components of the computer to run real time ie cpu, ram, gpu. If you run a game in a system without enough ram the computer would pagefile to the hard drive it and it would be a lot wors than just getting low fps.
Most games will not benefit from over 4 gigs of ram unless it is texture intensive such as flight sim games.
This of course is a lot different to how other software manages memory. For example photoshop and adobe premiere will use as much ram as you can throw at them with improving results because they use the ram as a incredibly fast cache for their calculations.
The recommended amount of ram for Global Offensive is about 2gb. I have yet to see it use more then 1.5gb while running.
More ram would help though if you like to keep things open while playing and even then 16gb would be over kill.