Get by id queries are majority when the database is used as a key-value store, and the only reason to such an underuse of RDBMSes is precisely because under heavy load some of them have hard time doing proper queries, with joins, subqueries and all the stuff databases are designed for.
It's not "underusing" an RDBMs, those are just the lions share of queries in many applications. Also note that a join say from some parent item by ID to children who have a parent_id makes essentially two lookups by ID in two indices, so the same reasoning applies; this is not just SELECT * FROM bla WHERE id = :x.
