The reason is because the priest should still absolve as long as this is possible. If there’s no time, then Anointing/Unction would suffice, but if there is enough time, then the priest should absolve. But he shouldn’t do that anyway. It’s the other way around. If death is imminent, the Church’s preference is that the sick person should (confess and) be absolved first, then if there is time, anointed; rather than anoint and if there is time absolve.
The Sacrament of Anointing/Unction by itself doesn’t have exactly the same effect as Reconciliation (Confession), because although it obtains the
forgiveness of sins, it doesn’t include
absolution. That’s why when a priest is confronted with a true emergency and death is imminent, he should say the short form of Absolution first, then immediately Anoint.
Note that the Catechism (1532 that was already quoted here) says that Anointing has the effect of:
- the forgiveness of sins, if the sick person was not able to obtain it through the sacrament of Penance;
That phrase “if the sick person was not able to obtain it…” is an essential element.