It is a result of the constitutionally defined separation of powers–and no, the debt ceiling is not unconstitutional.
Article I, Section 8, 2nd clause of the U.S. Constitution reads that one of the enumerated powers of the U.S. Congress is to:
The executive is vested with no innate constitutional authority to issue new debt separate from Congress.
The debt limit is actually an expansion of executive power. Historically it did not exist. Before its existence, every penny of borrowing done by the government had to be specifically approved tranches of specifically allocated bonds and such.
For example say the Federal government wanted to build a canal, they’d issue XYZ Canal Bonds at 20 year maturity paying % or what have you. This would have to be tediously done for every penny in the red the government had to operate. However aside from the Civil War, up until WWI it mostly worked fine because the Federal government didn’t do that much, and the debt outstanding tended to stay fairly fixed. There were problems with the system, but it mostly worked (during the Civil War a compliant Congress just issued a whole fuck ton of war bonds.)
In the build up to WWI the Wilson Administration and his treasury secretary basically made the argument that the way U.S. debt was structured and how issuance was handled was too laborious and procedurally difficult to handle the needs of a modern state. So what they came up with was basically the idea that Congress would no longer actually approve most individual issuances of debt for XYZ thing, instead they’d just allocate a basket of pre-approved debt, say $500m, and it could be used for whatever Congress appropriates it for later. This actually smooths things over because it’s a lot simpler with how treasury bonds are issued and how Congress operates, to just have that pre-approved debt that the Treasury Secretary can issue as needed.
Note that it wasn’t a huge hit to Congressional powder–the executive is still not allowed to spend money without congressional approval, so everything is still appropriated and budgeted by congress. But it adds a lot of flexibility for them to not have to issue new series of treasury bonds for every new spending. Note that they still issue individual bond series from time to time (they did it repeatedly during the World Wars, they did it after the 2009 Recession with the Build America Bonds and etc.)
Another thing the debt ceiling functionally allowed for was operating cushion for the Secretary of the Treasury, who could “cycle through” outstanding debt. Before the existence of the debt ceiling for example, all the different existing series of bonds were kind of locked in place with little flexibility. With the debt ceiling the Treasury Secretary can do things like use his pre-approved borrowing authority to say, retire some old series bonds and issue new ones, sometimes to the government’s advantage. That would’ve required special legislation previously. Now that power at Treasury is part of how they routinely do business.
Now in a sense the weird quirk is that we do have part of our constitution that says the validity of the U.S. debt should not be questioned (understood to mean it must always be paid back), and we also have an appropriations and budgeting process that is separated from debt issuance, which means we can appropriate and budget spending that we haven’t approved debt issuance to cover. Which means when we run up against that limitation, the President is put into a position where he can violate the Constitution by not funding things Congress has ordered him to fund to make coupon payments on the debt, or he can violate the constitution by letting the debt default and still trying to follow Congress’s appropriations mandate.
It isn’t great, but it doesn’t make any aspect of it intrinsically unconstitutional, just poorly designed for the times we live in.