And where does the Constitution grant them the power to do so?
If the federal government has not been specifically granted a particular power, then the federal government does not have that power; it is retained by the states, or by the people.
The right of a people to choose their own form of government is argued quite effectively and eloquently in the Declaration of Independence.
Article III, Section 3 of the Constitution grants the federal government the power to ADMIT additional states to the Union, but nowhere in that document is the federal government empowered to create new states.
Exactly how much power do you want to give the federal government, anyway?