Ehh, I know that the manga is literally the word of God, but we need to make interpretations of our own.
I've personally rarely seen an A-rank jutsu get copied. The only instance I can think of is Kakashi copying Zabuza's water dragon technique, but it's up for debate whether or not he copied it or if he knew it.
I know for a fact that Sharingan makes it easier to copy jutsu, but Itachi doesn't have the lightning element. The Sharingan has limitations, otherwise Sasuke would have used Izanagi when Madara stabbed him since he saw Danzo use it (Itachi in the manga stated that the Uchiha clan used Izanagi, nobody other than Danzo, Madara and Obito are confirmed to have Hashirama's cells as well as the Sharingan, so we can deduce that Izanagi is not just Senju/Uchiha jutsu).
The Sharingan also cannot use a summoning jutsu for an animal they haven't made a contract with. This would likely indicate that at least with some jutsu, there are prerequisites (such as actually knowing how to use an element).
Another thing is just that we take this manga as a bible, and kind of forget that Kishimoto is a human. He probably just didn't really think about it when he was drawing Itachi's fights. We give these characters too much personality when they are just man made creations.