Le pourquoi de sa méthode repose dans cette citation:
"""
In general there are two ways to build AMIs: Spinning up a new instance, customize it and create a snapshot or build the AMI from scratch.

The first option seems to be the most common case. There are several tools like packer or Netflix' aminator but since it requires to actually boot a existing AMIs, it has a few downsides. For once it requires a existing AMI which might include things you don't need but I'm more worried about the fact that it requires to actually boot a machine. This breaks the clean separation of build- and runtime and might mess up your build artefacts. Think logfiles, dhcp/cloud-init configuration and so on. Sure, you can clean up those things but I prefer avoiding it in the first place by never entering the runtime before actual deployment.
"""