Give monsters max HP, for their HD. Give the player characters the max HP, for their HD. That way, it's still balanced, and you won't get killed by a spitwad (unless you're Morgan.) There's enough extra randomness with the Constitution score.
No, it unfortunately doesn't work that way. Everything in D&D is balanced around average HP values: weapon damage, critical hit multipliers, spell damage, etc.
Changing that requires a lot more work than just giving creatures full HP as well, that would accomplish nothing but:
a) Make battles unnecessarily longer
b) Make it a lot more difficult for low levels
c) Make casters slightly weaker (actually a good thing, but not worth the trade-off in my opinion)
There are also effects that are specific to the NWN engine, such as having to manually adjust every creature's CR in the toolset as NWN seems to calculate CR partially based on HP.
For example a standard skeleton in the toolset has a CR of 0.5 and 6 HP (D12 hit die for undead), raising the HP to 12 (maximum for a 1 hit die skeleton) will raise the CR to 1. While it can then be lowered manually to 0.5 again, the skeleton (if employed in the same numbers as before) will be much more of a challenge than it is supposed to be.
Using maxed out HPs for player characters in a heroic setting where the PCs should feel epic and powerful from the beginning might be ok, in a setting like Ravenloft where you should have to constantly fear for your life it would be rather detrimental to the atmosphere.