Most games come up with their lighting implementation and their shadow implementation. Due to deferred shading, we are able to simulate lights independent from the scene geometry. But realistic shadows are still a big challenge in game development. They would have to overlap with each other, have soft edges and are calculated in real time to support dynamic scenes.
There is a presentation where the author, Chun-Fu Chao, mentions this about lights and shadows in game development.
1000 lights without shadow is easy, but 100 lights with shadow is impossible now.
I wonder where is the problem at all? I understand that shadow maps and shadow volumes have their problems though are very popular. But from my physical understanding, shadows are just the abscence of light. So if we use a very dark ambient color and use deferred shading for handling all light source with easy, wouldn't that result in shadows for free?