I keep hearing stories about how it's a shared engine and 'developer [x]' had to add X feature to make their game work....but devs are still adding things other devs had to 'solve' earlier...
Right but if you're creating say an Uncharted style 3rd person game, the tools that might be created by say BioWare from Dragons Age, those might not be the things this dev needs
Just cause Dragons Age is 3rd Person doesn't mean Uncharted will be easily replicated. Hell DA can be First Person or Isometric also if you go further into it, but games can differ, but tools can be really spread out
Just look at Frostbite, the hilarity of the engine when you try to do 3rd person or the glitches or bugs that arise in say Battlefield
Legs inverting, Necks getting long, the engine wasn't built to be used in that specific way
You have to have a dedicated team who job is to keep working on Frostbite
Create tools or assist devs in house about what they might need or help with