Apart from exclusive runes, most players obtain every rune in the game after several hours.
If there is RNG on runes, it will make finding runes more exciting, not just, oh another drop to recycle.
For example, instead of fixed 50% more dmg or 100% more chance, they can be reworked to like 35-65% more damage or 80-120% more chance or whatever you see fit.
Right now runes are pretty much the only drops that don’t have any RNG in them. (cards are fine, they are crazy rare)
Siralim is all about a never ending hunt after all.
This will also give some incentive to the Survey Ruins mission. If we don’t count the exclusive core, the mission rewards are obsolete at the moment. The way it is now, people do them for the core and don’t care about the main rewards.