Why is direct x not covered by Windows update?

DirectX has been included in Windows since before the release of XP.
The last version of Windows to be released with Dx9 was XP - Vista had Dx10, and Win7 has Dx11. Win8 has Dx12
All are backwards-compatible with Dx9
The problem is software related - earlier software may only be able to cope with the Dx version it was designed for. You may be able to run such software in Compatibility mode - which will switch the Dx version back to the one appropriate for that OS.

There is no point in attempting to install Dx9 in a later system - all you do is run the risk of corrupting that system.

NoelDP said:

DirectX has been included in Windows since before the release of XP.
The last version of Windows to be released with Dx9 was XP - Vista had Dx10, and Win7 has Dx11. Win8 has Dx12
All are backwards-compatible with Dx9
The problem is software related - earlier software may only be able to cope with the Dx version it was designed for. You may be able to run such software in Compatibility mode - which will switch the Dx version back to the one appropriate for that OS.

There is no point in attempting to install Dx9 in a later system - all you do is run the risk of corrupting that system.

Is this something that you have intimate knowledge of?
Because dx 9 is actually at version 9c, and has in fact received revisions and updates that some games and even graphics drivers look for now. These changes have occured since the release of windows 7, in fact several were made last year.
The way dx9 is handled in vista and win 7 would be more akin to an emulation mode. You still want the updated files though.

Direct x has not been so static since version 9 went up to 9c , and then continued to receive updates afterwards. Actually come to think of it, there was an 8.1 if memory serves to.
Rule of thumb for video/graphic editing, and gaming is generally to have everything even if you aren't using it. I have physx installed on my ati system. Why? because it makes certain games load faster when they check and see the files there. In a couple of cases it solved crashes.

This is the obligatory sentence I now spend making a case against proprietary ahem, bull substance software, like what nvidia has pulled with physx

That's getting a tad off base though.

From the download page for the Dx9c update.
<quote>
Note that the DirectX Runtime (Direct3D, DirectInput, DirectSound) is not part of this package as it is included as part of the Windows operating system, and therefore cannot be installed or uninstalled. Updating the DirectX Runtime is achieved by installing the latest Service Pack or obtaining a newer version of Windows.
</quote>

Like I said - you don't.
Last time I installed a Dx9 update was the Dx9c update in XP - about a year ago, on a fresh install.