Major corporations are not paying exorbitant licensing fees to have checksumming enabled by default. In fact, for enterprises running things like vSAN, ECC DRAM, etc, database checksumming is probably nothing more than additional overhead.

Database defaults in general are a touchy topic. Whatever set of defaults are chosen will be suboptimal for almost any serious user. A far more serious issue is figuring out the actual behavior of a database in different configurations. For instance, Oracle's SERIALIZABLE transaction isolation level only offers snapshoot isolation.