There are no spawn eggs for giants and illusioners

Just a note, they are considered unsupported mobs, so they probably didn't add them to prevent people from thinking they get supported. (Just like how the giant getting zombie AI and NBT was reverted to prevent people from thinking they were getting supported).

That said, if they remain unsupported, it may be an idea to make them part of experimental data packs.

Zombie horses are also unsupported and they have a spawn egg. I'd say that this is a valid inconsistency.

Relates to MC-189872 and MC-188666

There's also the Killer bunny that doesn't have a spawn egg

That's a rabbit variant, not a separate mob. Only separate mobs get separate IDs.