...you install a new (new to Steam) game?
I mean, who is gaming on Windows and doesn't already have DirectX installed? Surely the better option would be for the application to check first and then skip installing it. Same goes for it always installing some flavour of MS Visual C++ with games too.
it installs required libaries
Direct X is vast in size by default it installs what it required to run most applications. Some games use updated libraries that are not on your computer, which why they are installed. why install everything & bloat direct X & your OS?
Microsoft c++ libraries: Most games use c++ libraries provided by MS. each year MS improve these libraries with new features & can alter the way things are done. why do you need to install all not the latest? because
Games from Various years are built and complied on A particular library set. games developed in
2008 might use 2008 Library set
2011 might use 2010 Libary set.
The developer would need to edit their libraries links & re-test everything that makes a library call from the newer versions.
Also imagine if Libary set 2018 Broke support for X libary calls for a game developed in 2009. Developer is not going to fix what is not their fault.
Its Safer, Smart & potentially HDD saving, than Installing everything by default