Why dedicate an entire ec2 instance to a single container?
You would scale the virtual machines to try and reach optimal resource usage rather than pick a standard fleet of virtual machines for all workloads and let a scheduler do some kind of knapsack scheduling based on available resources.
Meanwhile containers enable you to run multiple services on the same VM, scale them horizontally as you need on the same pre-determined amount of resources, use blue/green deployments to spread your services throughout your VMs automatically, and achieve all of this automatically and effortlessly.