A post to the Steam Users' Forums by Valve software engineer John McCaskey answers a question about why Steam installs DirectX/.Net/Direct3D updates with so many games, even when a user's Direct3D is already up-to-date (thanks reddit). It turns out these updates are not updating DirectX or Direct3D itself, but are necessary to keep multiple flavors of D3DX libraries straight: "Each game that uses the D3DX helper library is linked to a specific version. As such the game must run the correct D3D installer version that it was specifically compiled with to ensure the binaries exist. Even if a later version of the binary is already installed, that version cannot be used, and even if your DirectX install is up-to-date because you've run a more recent version of the installer that is not guaranteed to have installed all previous versions. Even worse, if a version is installed for x86 it doesn't guarantee the same version is installed for x64, so 64 bit and 32 bit games may need to run the same exact installer version but targeting different platforms when run. Furthermore, Microsoft's licensing terms prevent anyone from distributing the files directly, the only way to distribute them is to run the installer, that's also the only supported method from Microsoft to check that the correct version installed." He goes on to explain the few options available to Valve to program more efficiency into this system:

The one thing that could be made better on our side is that Steam could be smart enough to know if an exactly matching version of the dx installer is already downloaded and share that content so you don't download it with each game. Since the installer is relatively small compared to most game installs that wouldn't be a huge win though and requires a good deal of new complexity for partners in how they package up their games and manage installation dependencies. You'd also still end up with lots of different versions of the installer, since as discussed above they are often targeting different D3DX versions and as such are all required. As such any improvement to avoid duplicates isn't an immediate priority, but we may be able to improve it slightly in the future.