There is no way to legitimately hatch most legendaries. Most legendaries can't even breed.
The only exception is Manaphy, which is transferred from Pokemon Ranger into Diamond/Pearl/Platinum as an egg, and hatches in the normal way. However, breeding Manaphy only results in Phione, so technically you still only get one Manaphy, no matter how much you breed it.
I'm pretty sure when someone requests a L1 Dialga/ Palkia/ Giratina, they want the ones obtainable in HGSS in the Sinjoh Ruins, where those 3 legendary dragons were obtainable at L1 if an event Arceus was obtained.
As to why they request other legendaries at level 1-10, either they didn't look properly at what they requested, or they want non-legit ones.