ACID has four letters for a reason. Atomicity is logically independent of
durability. A decent database will let you turn (synchronous) durability
off while fully guaranteeing atomicity and consistency.
The reason is that with a typical rotating disk, any durable commit is
going to take at least one disk revolution time, i.e. about 10 ms. Single
threaded atomic (but not necessarily durable) commits can be issued a
hundred times faster than that, because no synchronous disk I/O is required