People are not compensated as well as other top companies. If they had paid well, retention would be higher. There are many people who wanted to work for Amazon and did not negotiate its salary when joining as they should. Many of them assumed that if they performed well, it would be compensated with raise. However, the raise is very small. The company started to give more RSUs which is a good sign. Even with these changes, RSU and cash bonus are not as competitive as other online companies.
Another is, if you are not used to fast paced environment, you will feel left out.