You can, certainly create an AMI preconfigured with all of your libraries, packages and services ready to go, but the issues arise over time with package updates, system updates, one-off configuration changes .. the list goes on.
The actual configuration of your system will drift further and further away from that templated AMI, leaving you to constantly have to build a new template every time you deploy a new machine, or manually make all of those differential changes to your new system.
Config management can be tailored to rapidly update the configurations of certain classes of systems at a greater interval than the constant cycle of blowing away machines, updating templates, redeploying, etc , etc.
The actual configuration of your system will drift further and further away from that templated AMI, leaving you to constantly have to build a new template every time you deploy a new machine, or manually make all of those differential changes to your new system.
Config management can be tailored to rapidly update the configurations of certain classes of systems at a greater interval than the constant cycle of blowing away machines, updating templates, redeploying, etc , etc.