You can see what is installed in the base image here - https://git.cloudron.io/cloudron/docker-base-image/
Many of the docs/tools like file manager, terminal etc rely on the base image. For example, tar/zip availability. Ultimately, the images are optimized for developer and support time and not disk size. If there is a problem in an app, you want your tools there to be able to debug quickly. The base image is shared across all apps and services (databases), so it's a one time cost. I remember the last time I used an alpine image, it didn't even have ping to debug.
On a side note, you might see a little more docker image use right now because we are migrating all apps to a newer base image which is based on jammy - https://forum.cloudron.io/topic/8130/docker-base-image-4-0