Why doesn't OS X require graphics card driver updates?
Apple makes its own drivers &amp; they are included in the OS updates.

The way to be up to date on drivers is to always be running the latest OS.

There is debate over whether Apple make the entire driver set themselves, or just take NVidia/AMD's core code &amp; add their own hardware/OS-specific code to it. The latter seems more plausible to me, but either is just speculation.

The end result, though, is that there are rarely issues with Apple graphics drivers &amp; regular software or games; again speculation, but for games I imagine they write to a pre-defined rule-set, rather than try to constantly be pushing the boundaries.

As noted elsewhere, NVidia themselves make OS X drivers for Quadro cards but these are aftermarket cards anyway, not supplied in any Mac build option available directly from Apple.

Apple provides core drivers, currently up to OpenGL 4.1 or 3.3 - depending on your card, of course - plus legacy drivers for OGL 2.1 compatibility. Which one you get to use in any particular game etc is up to the writers of that game.  

Traditionally, if your card has full OpenGL support for the first monitor, it will also have it on the second, unlike many PC cards with 'clipped' performance on the 2nd monitor. 

Link to OpenGL support by Mac model

After comments &amp; other answers here…
NVidia do have their own release for certain Mac GPUs - this is the current driver as of December 2015
However, they are predominantly for the Quadro series of aftermarket cards rather than default cards supplied by Apple.
There is also support for some cards which have been legacied by Apple - namely the GeForce 680, 285, 120 &amp; 8800
There is mention of beta support for some recent mobile cards, presumably as Apple are dropping legacy support on these in favour of AMD in the current lineup.