Because standard libraries in mainstream languages are name-squatting on 'immutable' pretty hard.

You wanted 2+1 to yield 3, but instead you get a runtime exception telling you that 2 can't be changed.

I still don’t understand why they’re referred to as persistent vectors rather than immutable vectors, but I digress.

I believe that immutable just means, well, immutable, but persistent means that updates are achieved via structural sharing, so they’re efficient.