Here are some explanations on why you can't spawn some mobs in Minecraft.

Enderdragon: They will somehow cause too much destruction. It may also mess up the world, showing two health bars, instead if one It might also cause major glitching if you already beat the boss, since there is one dragon per world. (Maybe not in a super-flat world.)
These are only GUESSES!

Snow Golem: You can already create them, no need to have an egg (unless your lazy! :P).

Spider Jockey: They are two separate entities.

Giant: they are unused and can only be spawned by commands (that I know of).

Iron Golem: You can already create them, no need to have an egg.

Please DIAMOND this and COMMENT too!