Why can't I just use the prerequisites tag in the pom?

The prerequisites tag was designed to be used by tools like plugins. It will work for regular projects, but it isn't inherited to their children. If it is set in a parent reactor, then Maven will do the check. However if one of the children are built, the check is not performed. The enforcer plugin is designed to allow centralized control over the build environment from a single "super-pom", and to allow greater flexibility in version specification by supporting ranges.