Imagine the following exchange. 

Gov, why are landlords not paying this tax on their rents. Ans. Because they are providing a valuable service, housing. 

Gov, why are trust fund recipients not paying this tax? Ans. Because that would upset their parents and grandparents who created the trust fund in the first place. 

Gov, why are royalties from the oil industry not taxed? Ans. Because we want to encourage that industry. 

Gov, why are the wealthy who live off of their unearned income not taxed? Ans. Because we don't want to discourage savings.

Gov, why are high income wage earners taxed at the same rate (or less) than middle income wage eaners. Ans. Because we don't want to penalize success.

Gov, why are taxing only wage earners? Ans. Good question. Let me get back to you.