Most drops will be crap because you are comparing them the best drop you've gotten so far.
Roll a die 50 times. Most rolls will be lower than the best roll so far, simply because you're going to roll maximum pretty soon; then all other possible rolls will be "bad".
The only way to get away from that is to constantly increase the maximum and minimum roll. You could for instance add the number of rolls you've made so far to the die roll. But even then, unless the increase per roll is high compared to the variance, most rolls will still be low as it takes many increases for one good roll to be negated. (I won't do the math now, but I suspect +1 per roll is still in the "most rolls are lower"-category.)
In Diablo et cetera you will of course continuously get better loot, as it scales to your level, but it doesn't improve fast enough to cancel this effect.
It is possible to design a "loot-curve" such that new items are better than old ones, but there are issues with that. You have to ramp up the power level of items quickly, or have very little randomness in how good each item is or give out loot only rarely.
As to why such loot-curves are rare: they don't suit a majority of games.
First of all, it's hard to rate different abilities against each other, even a two stat weapon, speed and damage, is hard to rate, given preferences of players, interaction with armour et cetera.
Secondly, most games expect a long playtime, so power gain must be spread out over a long time. Nethack, in my opinion, has mostly useful loot; but has short playtime and your powerlevel goes from worthless tourist to demigod capable of killing Death.
Third: such a loot system isn't very interesting. http://progressquest.com/ is a good example of such a system. There are no choices to make: anything you pick up is better than what you had before.