Because that doesn’t make sense. Sapphires are blue. They are both corundums.
You can ask why don’t they just call a ruby a red corundum.
Why don’t they call a sapphire a blue corundum
From the Geology website:
“Red corundums are known as “rubies,” blue corundums are known as “sapphires,” and corundums of any other color are known as “fancy sapphires.””