Making divine smite a pre-emptive thing that you pre-load would drastically diminish its flexibility and lead to all of the exact same problems that auto-reactions have - you paladin will end up burning their level 5 smite on a 5 hit point goblin zombie minion that provoked an opportunity attack, rather than on the lich they intended to use it on. That should never happen, ever, full stop; it's a massive weakening of the ability.
