The flavors have to blend in with the other parts (PG/VG/nic) on a (almost) molecular level for the taste to come out really nice. When the flavors interact on that level the overall taste of the
juice alters. And that takes some heavy shaking and time.
Not all liquids need steeping. For example fruity juices can be vaped after just a few days (others immediately), while most
tobacco blends and other complex mixes benefit from a good long steep (several weeks - months).
Steeping (blending the flavors) is helped by warming the liquid, making it a lower viscosity and thereby easier for the flavors to mix. Try putting the bottles in warm water (50-60C), shake and put back and repeat. And then let them just sit in a closet.
If you want to do an ultra-steep (two weeks of steeping worth in only 1 hour) and you have an ultrasonic cleaner, try putting them in there for an hour. Especially handy for impatient users (like i am)
