I can`t to tune access to dictionary tables through the roles

My question is simple. After I grant access to dba_triggers as "grant select on dba_triggers to USER". User able to do "select * from dba_triggers"

But when I create role(select_dba). Grant to role select priv("grant select on dba_triggers to select_dba"). Then grant this role to user (grant select_dba to USER). After these operations user can`t to do "select * from dba_triggers".

Why?? I tried to alter this role as default.