Why is agreement in the contract not enough of a mechanism to make automatic offer be the case?
You can contract to sell me the White House. When the times comes for settlement, though, you'll be in breach, because you don't own the White House and so can't pass it on to me. And in the period between contracting and settlement, your promise won't give me any rights over the White House, again because you have no property in it that you can deal with.
You can't contract to sell yourself into slavery, either, as that is not a legal relationship recognised in Australian or American law. (In the case of the US, its an express constitutional principle, set out in the 13th Amendment.)
An offer isn't a "black box" or a legal fiction. It's an actual legal state of affairs, in which one party has rendered themselves liable to become contractually bound by another party accepting their offer, because their minds meet over the bargain.
Anyway, I found this case that engages with the relevant CC provision:
Great Minds v. FedEx Office & Print Servs., Inc., 886 F.3d 91 | Casetext Search + Citator
But it doesn't analyse it, because the plaintiff was relying on the effectiveness of the automatic offer to try and hold FedEx accountable for breaching the terms of the licence it had agreed to. Whereas the court found that FedEx was simply an agent for the actual licensee, being paid by that actual licensee to print the licensed material without itself having become an independent licensee. And there was no suggestion that the licensor wished to cease making offers.