A Republic's Government Is Not, And Must Not, Be A Business.
The United States is established as a republic. In a republic, the role of the government is strictly limited to those activities which are necessary to protect and foster the wellbeing of common infrastructure, broadly construed. Republics are essentially a form of trust (in the property sense) with the government of that republic the trustee. Trustees have fiduciary responsibilities - they must hold the wellbeing of the beneficiaries (the public at large) above their own. Profit motive is antithetical to those responsibilities and creates a conflict of interest prima facie.
Nationalization of infrastructure for the purpose of producing a sustainable stream of revenue necessarily involves a deviation from the role of trustee. The needs of the revenue stream come to take a superior priority to the beneficiaries' well being whenever they are in conflict (or the revenue stream is unsustainable).
Because it is necessarily limited to only that collection of revenue that is defensible in the eyes of the electorate and necessary for the fiduciary to carry out its mandate, taxation avoids the need for a profit motive. This is why government corporations don't turn profits except by accident.
Use fees (ticket prices, postage, licensing fees, etc) often get mistaken for sales as if the issuing entity were a business because of the format they take and how they are warned, but their purpose is not to derive profit but instead to defray the costs of providing the service and also to more accurately target users of a given system for taxation.
It is true that the government, especially at the Federal level, virtually always has an investment portfolio but its contents are the result of various policy failures and are transitory in nature; the government will seek to liquidate them at the earliest opportunity that benefits the citizens of the republic.
TL;DR Profit motive has no place in the governance of a republic and runs contrary to its definitional principles. Nationalization for purposes of revenue necessarily requires the gov't to take on the role of profit-seeking businessperson, which creates an intolerable conflict of interest.