"RNG stands for random number generator and is often used in various situations pertaining to card games, as well as games that also revolve around some sort of outcome. The reason for the implementation of RNG is to eliminate the option for players to cheat the system, allowing for more wins and racking up all of the rewards. RNG throws out a series of numbers into the air, and whatever it lands on that’s what is generated into the game and from there the party begins."
Just because you're doing more casts doesn't mean the percentage is going to get higher. Your chances sure. Percentage no. That's not how it works.
I've been farming for Sulfuras, Hand of Ragnaros with Sulfuron Hammer in my bags for a while now. I have 33 Ragnaros kills - have not seen it drop once. I do have Old Crafty and Ironjaw, though.
Wellp, I've heard of people that got raven Lord on their 3rd kill, I got mine after about a year.
Chances to not have that drop in 4000 casts are very small, but not 0, it's never 0 and it's never 1.