nested loop and merge join are disabled by default in legacy planner?

[Paul Guo]

  {
        {"enable_nestloop", PGC_USERSET, QUERY_TUNING_METHOD,
            gettext_noop("Enables the planner's use of nested-loop join
plans."),
            NULL
        },
        &enable_nestloop,
        false, NULL, NULL
    },
    {
        {"enable_mergejoin", PGC_USERSET, QUERY_TUNING_METHOD,
            gettext_noop("Enables the planner's use of merge join plans."),
            NULL
        },
        &enable_mergejoin,
        false, NULL, NULL
    },

I just checked greenplum/gpdb. The two guc are disabled by default also.
Anyone know the reason or history? thanks.