H-1B workers are paid equal to or higher wages than US Workers because their wages are literally
dictated and enforced by the Department of Labor, which require high salaries in order to dissuade the displacement of American workers. It is therefore more expensive to pay for an Indian H-1B than it is to pay an American worker, because with an American worker, you can pay them whatever you want, as long as it is above minimum wage.
Go here for the Federally mandated wage library, wherein wages for foreign (H-1B and Green Card) workers are etched in stone, and as you can see, are generally higher than what most American citizen workers in this country get, for any type of job.
Before an H-1B can work here, the employer has to have a labor certification approved first, and compliance with the DOL wage standards are mandatory.