Why Deck Building Templates are the same
Your premise isn't entirely correct. The template of playsets of about 5 creatures and about 4 spells is a pretty good description of an aggro or midrange deck, because that matches the aggro and midrange gameplan. But that's not the only kind of deck. If you look at MTG Top 8's current Standard metagame breakdown, you can see that aggro decks are in fact relatively dominant in the current metagame. But there's also a significant number of control decks, which have a different game plan, and different decklists to match.

This blue/white control deck, for example, plays 11 different non-creature spells in varying quantities, and only two copies of one creature, which are likely there more for the utility of its ability than for the creature itself. This blue/black/red deck follows a similar plan in different colors. On the other hand, this God-Pharaoh's Gift deck plays a lot of creatures and no non-creature removal spells, because it has a different game plan.

Even if you look at the Aggro category, there are more than 10 different broad archetypes of aggro decks in the current standard meta, and each of those archetypes has a variety of matching decklists.