I understand that games require it to run, but shouldn’t having the latest version cover all previous versions? Why do you have to install the year specific to the game each time, even if you already have games with that same year installed?
The reason is that there are many different versions of DirectX and each one has slightly different features and implementations.
If you tried to run a game on the wrong DirectX version, you would most likely encounter errors or glitches in graphical processing.
So, in order to avoid these problems, each game will come packaged with (or otherwise install) the specific DirectX version that it was designed for.