Launching products is considered more important than ensuring that code is robust and documenting how things work.
Despite Amazon's large size it doesn't have a lot of well developed work processes & groups often have different goals & missions. This makes getting things done for a project challenging since a team that needs to do work for your project might not think it's important and might not be able to accommodate your needs.
It's a pretty competitive and political environment. There are people who will take credit for your work (since they are looking for their next promotion) so make sure your keep your manager informed of what you are working on.
Most teams don't get it right the first time so make sure you get everything tested.