This reply would make more sense if you could provide a valid reason as to why all drop tables should work the same across all bosses all of a sudden.
We talk a lot about technical debt and design debt, and this is really what that looks like in practice.
Maintaining 10 systems takes 10 times longer than maintaining 1 system. Even more if some of the systems are significantly more complex, and probably longer still because complexity doesn't scale linearly. Maintaining unnecessary systems is bad because time spent doing that is time not spent doing something else - like making content.
It's worth maintaining multiple systems if they're important and hitting fundamentally different things - like you can't fold bosses and quests into a "single system" in order to make maintenance simpler. Drop tables, by comparison, are a relatively simple concept and while it's not that there's no possible benefit to having different bosses work in a different way, from a cost/benefit POV the tradeoff is significantly worse.