Question

Don't understand.

Answer 1

Just like Reshiram,Zekrom,and Victini have Shiny Forms, but they cant be obtained. It was put there by Game Freak for who knows why.

Answer 2

It's because if you hack or glitch, and get a legendary Pokemon egg, but if they don't have egg steps, your game can become ruined.
Makng "exceptions" in the system can cause those things to happen.

Comments

I don't think that's the right answer, plus, you don't have a source.

---

http://pokemondb.net/pokebase/77199/why-do-pokemon-that-cant-hatched-from-egg-have-base-egg-cycles
It is there to keep the game from messing up.