Because you generally want to deploy them independently. If you have 50 microservices and have to run an AMI build every time that's going to significantly slow you down. With containers you generally only deploy the service that has changes to a VM that doesn't change as often.

You want them to be independent, atomic. How they are deployed and the choice of technology you choose to couple them with? That should be driven by load -- and dynamic.

The reason everybody's so hot on microservices to begin with is that we kept coupling everything all together and it became a huge mess to manage. You don't want to repeat that mistake, only at cloud scale.