The problem is that there is an audience for whom exactly-once sounds like it makes their non-specialist lives much simpler compared to an at-least-once system because they can offload the necessary distributed systems expertise to somebody else.

People insist on this messaging precisely because that crowd is somewhat vulnerable to this sort of shenanigans.

It requires teaching the engineering team to reason and prove idempotency guarantees.

I've seen teams try to continue programming the way they always have and try to throw difficult problems over the wall. It is exactly-once in a non-threaded, monolithic, stateless web app, so why shouldn't it always be like that? It's part of a programmer's mindset to try to create abstractions and reason within a simpler problem space.