1. Adding layers, software, configs etc in your Dockerfile is potentially error prone. Adding a single package might not seem much but as soon as complexity is growing, OS tweaks applied etc, it comes in handy to have this done for you by a vendor/community in the base image.
2. Sharing as many layers as possible between containers improves resource efficiency. Image layers which are same between containers can be reused. So, if there's many application instances running on a host, it can have a real impact if they all use a shared openjdk base image with few additional layers, or just a shared ubuntu image with openjdk installed via Dockerfile (which might differ in versions, configurations, ...)

Disclaimer - did not find an official reasoning for this best practice, but this is what I came up with, hope it makes sense.