The legal definition of theft in most countries is based on someone (A) depriving another person (B) of a given entity. It is fundamentally the transfer of the entity to A which removes it from B.

It therefore cannot apply to an idea, for which A getting his hand onto the idea will not make B forget about it.

For copyrighted works, creating a copy is is not depriving someone of the original entity, so it is not stealing, but counterfeiting.

Along the same vein, you can't steal something from someone if he has not acquired it in the first place. Creating a copy of an entity and selling it or giving it for free might cause monetary grief to the original owner (which might sell his entity for less, if at all, than if the copy did not exist), but it is not stealing either.