Answer:
As the President of the Senate is the Vice President of the United States, who is elected together with the President to be in charge of the executive branch of the nation, if it had the same powers as the Speaker of the House, it would even surpass the powers of the President, since it would have full control over both houses of Congress. Therefore, the position of the Speaker of the House is intended in some way as a limit to the power of the Vice President, to guarantee the full authority of the President of the United States.
As a result, the power of the President as head of state and head of government is not contended by any other lower position within government.