Because Art. 2, section 4 does not include incompetence or mental instability.

Because The 25 Amendment, Section 4 requires "Whenever the Vice President and a majority of either the principal officers of the executive departments or of such other body as Congress may by law provide, transmit to the President pro tempore of the Senate and the Speaker of the House of Representatives their written declaration that the President is unable to discharge the powers and duties of his office, the Vice President shall immediately assume the powers and duties of the office as Acting President."

In either case, we would have to deal with Mike Pence, who wants an American theocracy, and that would be worse than Trump and his cronies.