German labour legislation is stacked against strikes. More importantly, the presence of works councils means that employees are apprised early of how a company is doing and able to negotiate with management before it comes to strikes. Everyone knows that strikes are not good for business so everyone tries hard to avoid them, or restrict them to "warning strikes" of a couple of hours.
The reason for the problems is that Amazon is only prepared to pay people in accordance with the logistics branch, which is a lower wage than the catalogue shopping branch. In addition a lot of the employees are agency staff from other countries in an attempt to drive wages even lower. Germany has a growing problem with its working poor being paid too little to live on.