Because it's impossible to make such things where anybody would be happy with the result.
There are systems which support standard C and C++, but they don't even have graphics (or a display). Specifying the burden of a graphics library would be a complete waste of time.
Plus, graphics capabilities evolve rapidly over time. By the time the standards committee agreed anything, what they would be agreeing to would be hopelessly out of date.
The best you can hope for is a portable library that works across all the systems you care about.
Such as https://www.libsdl.org/
> on Turbo C\C++ we have the Graphics library
TurboC was a vendor specific
implementation targeting a single platform.
Vendors include such things to attract customers, and it's a relatively easy thing to do when you only care about one platform.