Because they have always been intending to come out with codecies for 8th edition. They also don't have all the primaris units in the Imperium index or chapter specific rules or warlord traits or relics, because again those are all going to be in the codex.
Look at the index's not as a 100% replacement for the previous editions codex but more a semi stop gap that allows you to keep playing and the dev team to make tweaks and changes before giving you something more finalized.
Also, the in game difference between a chapter master and captain won't be all that much. Probably one extra wound and attack and maybe come clad in artificer armour.
A forum community dedicated to Warhammer, Warhammer 40K enthusiasts. Come join the discussion about Warhammer and Wargaming collections, miniatures, tactics, terrain, reviews, accessories, history, displays, models, styles, scales, reviews, accessories, classifieds, and more!