Backup and restore for PostgreSQL + why number of backups increased while we limit the recovery window of 7 days
Your listing of /barman/main-db-server/base suggests you are taking a backup once a minute (Why and how?  I don't know.  Your given crontab doesn't seem to launch a backup that often--but something seems to be).  None of those backups are more than 7 days old, so there is no reason for them to be deleted based on the retention_policy = RECOVERY WINDOW OF 7 days

But you should really list your backups using barman list-backup, not by manually monkeying around inside barman's workspace.