In theory, because there is some marginal benefit to you to knowing that you got upgraded early, in the circumstance (like your OB flight) where they're comfortable doing that. In practice, because the upgrade windows used to be more important and they've probably just left this "benefit" around to try to make higher status tiers seem more useful.

There's a long but somewhat dated thread on this topic: Has AA eliminated the upgrade window for elites?

Edit: The earlier upgrade window does also make it harder for AA Revenue Management (RM) to accurately predict the amount of demand in J, so you *could* get lucky and clear an upgrade at T-100 and then 12 people come along and (try to) buy J, but one of them can't because you've already been upgraded. This is obviously a very bad thing for RM though, and I suspect this happens very rarely in practice.

Edit 2: This does also mean that you could potentially get upgraded before someone who nominally has higher priority than you, if you get cleared at the window before they purchase their ticket, whereas if they held all upgrades until say T-2 you then wouldn't have gotten the upgrade. For example, suppose there was exactly 1 J seat left and you got cleared at T-100 as an EXP (this likely wouldn't actually happen in practice), but then a CK comes along and buys a Y seat; you would keep the upgrade and the CK would go without, even though CKs are supposed to have upgrade priority.