The docs say it pretty well:
When a Job completes, no more Pods are created, but the Pods are not
deleted either. Keeping them around allows you to still view the logs
of completed pods to check for errors, warnings, or other diagnostic
output. The job object also remains after it is completed so that you
can view its status. It is up to the user to delete old jobs after
noting their status. Delete the job with kubectl (e.g. kubectl delete
jobs/pi or kubectl delete -f ./job.yaml). When you delete the job
using kubectl, all the pods it created are deleted too.