It’s baked into the US Constitution and the framers intentionally made it difficult to ratify amendments to prevent it from being changed frequently on whims (the Constitution should ideally be able to weather any change in the breeze). He could but he’d need 2/3rds of both House and Senate to vote in favor in order to proceed, so he’d need those to be in session to vote, and then ratification is even messier, I think 2/3 or 3/4 of state legislatures need to vote in favor of ratification. Bunch of hurdles that require not only getting all these groups in a room, but also to have well over a majority vote in favor