For the real reason you need to ask the mod developers themselves. My guess why they did it? They want the resource to be scares and/or hard to obtain and thus using RNG can make a lot of sense and is a lot easier then some other options to do that.
Lets say you want item X to exist and be dropped from creepers. If you want to avoid RNG (which is technically already impossible as what spawns where is already random) your only option is to have creepers always drop Y amount of item X. Maybe not a problem for mundane things but if you don't want players to get too much of it you really want to reduce the amount of times it drops. This leaves you with some options
make creepers drop item z instead, which can be crafted into item a which can be crafted into item b... (repeat until you have the amount of steps you like). Don't do this, it generates a lot of useless items and staring at a crafting table all day to get what you want is not fun.
Use some counter that keeps track to how many creepers you killed since you last got item X. Mods break stuff, this sounds like something that can easily be broken either by accident or on purpose by mods, resulting in item X to potentially be unobtainable, become way to common or get messed up in another way. If the count is global (thus all players share the same counter) players may be able to setup systems that farm creepers so affectively that other players pretty much have 0 chance to ever get it.
Just use a random number. Due to how randomness works it can result in pretty much the same behavior as the second option, without being as easily broken by other mods as adding items to drops using RNG is a standard feature that mod makes know how to handle. Because of this it is also the easiest to implement.
Same with putting it in loot for dungeons and other buildings. The RNG causes the player to roughly need to find X of this building before he gets it.
