Just because the product is big doesn't mean its a monolithic codebase. Amazon is powered by a lot of different pieces. Decoupling let's you keep the pieces small and yet grow and scale to huge sizes.
It's also a maintenance nightmare. A LOT of that code is terrible, there's a lot of tight coupling, and a heavy reliance on developers with pagers to keep the mess operating.
There's a reason that they have such insane turnover so few people who stick around after they collect their signing bonuses...
