Because this game engine has a colorful history of bugs introduced by forcing it to change that setting. Oblivion made it far more obvious in the form of the bouncing landscape bug. Fallout was less obvious about it, but it routinely led to save corruption if it was used for too long. I strongly suspect Skyrim to be the same, but that's all anecdotal evidence at this point from people who tell me they have no problems and are set to the default of 5, vs those who rage on about my borked mods but turn out to have a setting of 7, 9, and one guy even had 11.
That seems a reasonable conclusion. My thoughts are that anything that pushes the game runs the risk of breaking something and upping the uGrids to load will certainly tax the game heavily. I do think is a combined effect though - of mods,load order, low/high end system and game settings. Basically its all put in the pot and if you have the wrong mix you end up creating napalm. What is or is not the correct mix is open for much speculation and debate.
With Skyrim it's even worse too because you can't reverse the change without destroying your save. Frankly I don't think Bethesda should continue honoring the ability to change that parameter because all it's ever done is bring misery to the process.
As a huge supporter of uGrids to load for increased immersion/enjoyment, id be against such a drastic measure. However I do agree that perhaps it shouldnt haven been published in the Nvidia tweak guide as a recommended tweak.