billions on this planet have never heard of them
"Between 1900 and 2000, the increase in world population was three times greater than during the entire previous history of humanity—an increase from 1.5 to 6.1 billion in just 100 years."
It don't matter; how to keep up with the ever growing population? And people heard of them, rejected the message, and then died leaving offspring behind, so would it be just for God to resurrect such people? What would be the point of the preaching?