But why do adult humans, who almost always have the ability to just buy the gifts, give and receive gifts?
I personally hate gifts, but I can imagine why people enjoy giving and receiving them. The reason is that it demonstrates that someone else cared, that they took their time and spent energy for you. Likewise giving a gift is an act of appreciation for the other person for the same reason.
But this also means that the idea of automating gift selection makes the whole thing redundant. What purpose is there in AI gift selection? It becomes just a stupid ritual, in which people spend money to fulfill some social obligation.