So, do hit points badly model reality, or do they cleverly model luck, divine favor, etc.? If the latter, why can't that luck help against anything except damage from attacks that hit?
It would say it is the former. It is all too easy to come up with some sort of damage track table, where a PC starts accumulating all sorts of penalties when his hp dips below a certain percentage, but it would just not be fun to be always fighting at a fraction of your actual prowess, plus all the bookkeeping can get tedious fast.
Wotc briefly touched on this in Rules Compendium. Basically, a balance has to be struck between realism and ease of gameplay. Hp clearly favours the later. Just keep taking damage until your hp goes below zero. Then something bad happens to you. Easy and straightforward. No need to worry about what happens when you have 10 arrows sticking out of your body, or when a giant bonks you on the head with his greatclub.
People started coming with all sorts of alternative explanations to try and rationalize the supposed abstract nature of hp, but I feel that this is one area where you should just close one eye and not think/worry too much about it. Too much effort for too little benefit, IMO.