1. Someone else maintains the software running these services, including OS upgrades, security upgrades and patches, uptime monitoring, etc.
2. Since every logical component is an independent service, each scaling independently, any one single component is unlikely to become a bottle-neck while scaling. In traditional monolithic servers, you'll have to have contingency plans if you beat storage/network/CPU/RAM limits
3. The closest thing to this is to break up a monolithic email server into microservices and deploy them as independently scalable containers, which is a considerable engineering effort.
Assuming this works as advertised, you can go from zero to a full blown email service for organizations with thousands of people (assuming the stated AWS limits are lifted), in record time