The question should be - why is it NOT when that's exactly how the Founders left the Articles of Confederation and Perpetual Union.
From the first legal treatise written after Constitutional ratification -
But the seceding states were certainly justified upon that principle; and from the duty which every state is acknowledged to owe to itself, and its own citizens by doing whatsoever may best contribute to advance its own happiness and prosperity; and much more, what may be necessary to the preservation of its existence as a state. Nor must we forget that solemn declaration to which every one of the confederate states assented . … that whenever any form of government is destructive of the ends of its institution, it is the right of the people to alter or abolish it, and to institute new government.
Consequently whenever the people of any state, or number of states, discovered the inadequacy of the first form of federal government to promote or preserve their independence, happiness, and union, they only exerted that natural right in rejecting it, and adopting another, which all had unanimously assented to, and of which no force or compact can deprive the people of any state, whenever they see the necessity, and possess the power to do it.
And since the seceding states, by establishing a new constitution and form of federal government among themselves, without the consent of the rest, have shown that they consider the right to do so whenever the occasion may, in their opinion require it, as unquestionable, we may infer that that right has not been diminished by any new compact which they may since have entered into, since none could be more solemn or explicit than the first, nor more binding upon the contracting parties.
Their obligation, therefore, to preserve the present constitution, is not greater than their former obligations were, to adhere to the articles of confederation; each state possessing the same right of withdrawing itself from the confederacy without the consent of the rest, as any number of them do, or ever did, possess.
Of the Several Forms of Government, St. George Tucker, View of the Constitution of the United States, Section XIII
Yes, there is a right of revolution. Our very nation is founded upon it.
However.
Unilateral secession must inevitably have recourse to the Laws of War. A seceding California must either purchase, leave alone, or seize all the assets within the State that were created by and belong to the rest of us.
Since they cannot afford to purchase the Navy facilities at San Diego, for example, and since we would be crazy to give them up, they must have recourse to the Laws of War to have them, and, once that dog is let out of its kennel (which, as you point out above, they have the Right to do), then they will have to abide by the result.
And the result of California warring on the United States is quite unlikely to produce the result the secessionists desire.