Because they are. Earl is right about the dumber part. South Asians are actually technically "white" too, they are also Caucasian.
In what way are South Asians Caucasian? South Asians have no substantial ancestry from the Caucasus and speak no Caucasian languages.
Today the linkage comes from the commonality of Indo European languages. Originally there was a classification that grouped ethnic groups based on cranial features, and a hypothesis that an original population was from the Caucasus, but that has been replaced by modern genetic studies.
By that logic, Middle Easterners actually shouldn't be considered White. Nor should Hungarians or Basques.
