Because Konami realizes that most players do not have the time or creativity to create their very own decks, so they release "premade" decks within BOXs to make more money.
Do you really want to buy support cards from 5 different boxes to build 1 deck? 🔥 no. Although they might start listening to you to get people to spend even more $ on this.
Now im thinking about that too... BLS ritual desperately need more support; the same goes for psychic, gemini, harpies, etc. And this would be profit for Konami as well since people will buy older boxes to play these old decks
...while on the other hand powercreeping older decks. Konami shoud change the way they release box, dont just giving us new archetype every month but also upgrade older decks by releasing more support. That way people still able to enjoy their favorite archetype/deck without being afraid investing on older boxes *looking concernedly at GB and SixSam*
Ehm in the last box we have support for Amazoness which are an old archtype and in this set technically there is support for spellcaster deck. So your claim is not necessarily true. They support both old and new archtype just that there is a limited amount they can release at a time so quit complaining.