If you've ever had the displeasure of seeing the sorry state of VM tooling you would have known that building custom VM images is a very complicated endeavour compared to podman build or docker build.

I once tried to build a simple setup using VM images and the complexity exploded to the point where I'm not sure why anyone should bother.

When building a container you can just throw everything into it and keep the mess isolated from other containers. If you use a VM, you can't use the OCI format, you need to build custom packages for the OS in question. The easiest way to build a custom package is to use docker. After that you need to build the VM images which requires a convoluted QEMU and libvirt setup and a distro specific script and a way to integrate your custom packages. Then after all of this is done you still need to test it, which means you need to have a VM and you need to make it set itself up upon booting, meaning you need to learn how to use cloud-init.

Just because something is "mature" doesn't mean it is usable.

The overhead of docker is basically insignificant and imperceptible (especially if you use host networking) compared to the day to day annoyances you've invited into your life by using VM images. Starting a a VM for testing purposes is much slower than starting a container.