Why did Abraham receive promises of a spiritual seed in the heavenlies?
The promises were given because those spiritual descendants would enter by faith plus nothing. That matches Abraham's salvation and Paul's later message, whereas Abraham's physical descendants were connected to Israel, the earthly people of God, described as the dust of the earth.
Les explains that Abraham had promises of a spiritual seed in the heavenlies because they were going to come in by faith plus nothing. Abraham himself was saved by faith plus nothing, and Paul later taught the same. Everyone in between, however, was saved by faith plus works, including John the Baptist's call to Israel to repent and be baptized. Abraham's physical relationship, by contrast, was with the nation of Israel, the earthly people of God, the dust of the earth. Romans 4:13 also says the promise that he should be heir of the world was not to Abraham or his seed through the law.