This is one of the issues with itemization within D4. In D2, if you needed faster cast rate, you can farm a pair of magefists as early as normal Diablo and they’d be your forever gloves.
Maybe it’s this way now because the devs don’t want max level builds needing to go back into an easier difficulty just to farm a specific item. However, this way items just don’t matter until level cap and this makes everyone want to get levelling over with as soon as possible.