problem is that its not always compatible with features you use on your production database
Or along similar lines you could divide it such that developers can test things locally on their machines with SQLite, but once it gets pushed into CI (and passes code review etc.) it's tested against the heavy db.
I meant in situations like parent comment where you're using an ORM such as hibernate that supports multiple databases, you can test as much of the non-DB specific stuff with SQLite in-memory and then do a separate batch of tests with DB specific behaviour.