Primarily because instances are disposable. Getting more traffic to your Foo service than you expected? Spin up another Foo, and let the load balancer figure it

Primarily because instances are disposable. Getting more traffic to your Foo service than you expected? Spin up another Foo, and let the load balancer figure it out. Then, an hour later, when traffic goes back to normal, kill the new instance. This is easier to do when your service is self-contained. It's certainly possible with an app server, but 12-factor apps tend to view the service as an atomic unit that can be started and stopped wholesale.