Because that could mean he could lift term limits and he could be the president until the day he dies.
That has been one of this dreams ever since he was installed in Oval Office the first time. He has been trying to become the "emperor" just as long. For example the chairman of the Joint Chiefs of Staff general Mark Milley described the last months of his first term by comparing it with the Nazis and the Reichstag Moment they pulled off, specially after Trump's goons attacked government buildings after the election and all the mess he caused back then...
Even recently a clip came out of him talking to the Zionist Zelensky and was fascinated about the idea of being able to remain in office even after the term ends, in case the country was at war!
Trump's attempt to halt federal payments in order to ensure compliance with his order could also be seen as another notable example that clearly shows just how authoritative Trump is and it wouldn't be wrong at all to say that Trump's approach to governance has been characterised by a willingness to test the limits of presidential authority.
And with clip about his fascination with remaining in office even after his term clearly shows that he has no commitment to Democratic norms and peaceful transfer of power, and honestly such actions undermines the principles of democracy and the rule of law which he clearly claimed adherence to before becoming president.