Unlike bees that are swarming and have prepared their wax glands to start flowing by gorging themselves on honey before leaving, package bees had no intentions of leaving their hive and have no prepared their wax glands.
The difference is like several friends packing their bags to go start a new life together on an island. Versus a bunch of strangers crash landing on a island. It takes time for a package to build up. If you can afford it and want some honey the first year, a nuc is the way to go.
Another analogy I like to use is going to a homeless shelter and randomly picking 6 people, putting them in a house and expecting them to be family.