The answer to this question is in the structure of the Union. The States were first, and the states together created the Federal government. This government is of limited powers, while the several States retained the balance of the power; another way of putting this is that the federal government has only the powers delegated to it by the states, while the States exercised plenary authority in all matters not delegated.
Keeping this in mind, the answer to your question is simple: the mere fact that the states chose not to delegate expulsion authority to the federal government (and could certainly act to expel a state without its consent) does not effect the State's authority (not delegated to the federal government) to leave the Union.
It has the power delegated to it by the Constitution, not the states, per the 10th Amendment.
Keeping this in mind, the answer to your question is simple: the mere fact that the states chose not to delegate expulsion authority to the federal government (and could certainly act to expel a state without its consent) does not effect the State's authority (not delegated to the federal government) to leave the Union.
But the Constitution does delegate the sole power to create states to Congress. By implication doesn't that mean Congress can also uncreate states?
And if you look at Article VII, then it appears that the states can. According to Article VII, ratification by 9 states was sufficient for adoption of the Constitution. So nine states could force the issue on the other four - ratify with us or take a hike. And if any of those states refused to ratify then they were out of the Union and out of the country. In effect expelled. So by rights it appears that states could be kicked out unilaterally. Right?