Why don't they make them different Patterns for them other than the ones they have now
It is just how Game Freak wanted it. All Pokemon eggs look the same Eccept Manaphy in the games.
In the anime the Eggs doesnt look the same.
^ Scroll down untill you see Other Pokémon Eggs in the anime*
Hope I helped
It's all part of the mystery. If you send the egg to a friend and don't tell them what it is they have that mystery. It also ties into what the Day-Care Man says.
I think eggs that look the same because it makes the game more interesting because you don't know what you will get.
Game Freak originally wanted eggs to match the patterns of their species, but it would have taken too much coding, and LegendaryTrainer has it right on how they wanted it to be a mystery to the player (although it would be pretty obvious if you understand the mechanics of breeding in the first place).
However, there are some "special" eggs that don't match the basic appearance, like the eggs of the Legendary Bird trio in Pokemon Snap. And, of course, Manaphy. I guess that those are the only official in-game examples.