COR isn't totally at the mercy of a random number generator though. They get abilities like Snake Eye to manipulate the outcome of "luck" to be more favourable.
THF is at the mercy of luck just as much as every other job, like Karbuncle explained above, THF just gets more roll of the RNG dice. If you look into reward systems, Luck is actually a very effective way of keeping people hooked on a task (i.e. keeping people playing).
Humour me by allowing me to describe some psychology. When people do things, they generally get rewarded, but the schedule of this reward has a big impact on whether the person continues to do whatever it is they're being rewarded for.
Two examples are what are called "fixed" schedules. These can be described in the way jobs and commission works. A "fixed interval" schedule is for example getting paid £10 an hour for working. A "fixed ratio" would be like commission, I.E you get given £20 for each 10 sales you make.
The other two are "variable interval" and "variable ratio". "Variable interval" works like random drug testing at work. "Variable ratio" is like playing a slot machine in a casino.
The most powerful reward schedule for getting someone to do something, is variable ratio. Any sort of gambling system works on a variable ratio schedule, because people think "oh but I might win this time!" and continue on and on into infinity.
Online games (or most games) work on this variable ratio system. You will keep going back and killing that NM because you never know, it might drop this time! Because of this "hook" the games have, they will never add a "luck" stat that you can improve and in turn, increase the chances of an item dropping (and therefore, reducing the "hook" effect to keep you playing and paying) UNLESS the method of increasing the "luck" stat is in itself an item you have to pay for with real money.
Sorry for the waffling post, but if you want to read more about it, you can check the wikipedia article for Reinforcement schedules here.