I think you're dramatically underestimating what a buildpack is.
The Java buildpack is a Ruby application with a modular dependency download/caching subsystem to enable both internet-connected and air-gapped enviroments, hundreds of unit and integration tests, support for runtime injection and configuration modification for TLS certificates, support for several languages including Scala , Groovy, Clojure, support for several package types, several app runtimes, etc. https://github.com/cloudfoundry/java-buildpack
The PHP buildpack is a Python application with a modular dependency download/caching subsystem to enable both internet-connected and air-gapped environments, hundreds of unit and integration tests, support for easily configuring many different PHP extensions, libraries and modules, and extension points to customize the container staging behavior. https://github.com/cloudfoundry/php-buildpack
For developers, this saves a lot of time, effort, and variance for staging their containers.
For operators, this enables a control point that simplifies CI/CD pipelines dramatically and standardizes a means of containerization across projects.
It might not be for everyone, but I think it's a useful alternative to Dockerfiles.