Database systems lock pages when writing to them, to maintain integrity. Using 1GB pages would cause excessive blocking in many if not most transactional databases.
In database engines that use page locks, the locked page size can be different from the file/mapped/allocated page sizes. If you still have excessive locking while using smaller page locks, there are other ways to reduce contention as well, such as CoW to protect concurrent reads, deferred write-merging to assist concurrent writes, and the storage equivalent of RCU.