The coding might be easy but the processing would not. That's a lot of calculations to do. The server would have to search each person's inventory then calculate new drop rates for each individual drop for each instance. Would probably require some hardware upgrades and a lot of development time to make sure it works well.

I'd rather they spend my subscription money on developing new content rather than reinventing the wheel. Every game has some form of RNG so if you don't like it stop playing video games.