They do? Every ORM I've used supports explicit transactions and rollbacks. EF, NHibernate, GORM, SQLAlchemy. What major ORMs deny you this functionality?
It isn't that there is no such thing as "a way to open/close transactions". It is that transactions usually break all the supposed ORM benefits like composability. Often, you cannot even method-chain transactions like foo.search().begin().insert().insert().commit(), you have to do it in multiple lines. Even worse, rollback handling is often an exception, breaking control flow of your application.
And most only support basic transactions, not checkpoints.
