Because when read in context this passage isn't talking about a nation of Israel.
Firstly, regarding salvation Paul never talks about a nation of Israel, I have yet to see anyone provide me any verse anywhere, NT or OT, that states "the nation of Israel is saved". It just doesn't exist in the Bible.
In terms of salvation he only talks about the people Israel. He very specifically makes the distinction that not all of Israel are Israel. He very clearly establishes the fact that not all of Israel will be saved, but all Israel will be saved. And he clearly alludes to himself as a member of all Israel and therefore evidence that the promises have/will be fulfilled.
It's an apposition. Individuals that meet the conditions of being a) his countrymen, b) his fleshly kinsmen, c) Israelites, d) to whom belongs the adoption of sons and daughters, glory, covenants, Law, temple service, e) the promises.
Not all children of flesh meet the condition of belonging to the promises. He clearly isn't talking about all of his countrymen, nor all fleshly kinsmen, nor all Israelites, but all of those qualities plus being heirs to the promises. Paul takes a lot of time to dispel the misconception that not everyone that claims to be heir to the promises is actually heir to the promises. The promises were only ever to those with an internal nature to turn to God and not all that were counted among Israelites.