Except that developers usually either don't care about the underlying issues of managing the OS, or don't have the skillset. They are focused on developing the product/features, not on ensuring that images have security patches, or even if the environment is secure is any way (file permissions, network ports, etc...). Those issues are large enough that they require dedicated resources (evidenced by the existence of professions in Operations), and moving them to a container generally just allows developers to ignore them -- not to take over the responsibility.