The problem is that with only 4 base classes and just kits, it would be too easy to mess up archtypes and break balance.
Lets the Ranger.
What if someone wanted to make a "3E style" ranger: a skilled light armored warrior with access to a few spells and some animal related ability.
The designers will have to design a way to:
- Change the Fighter skill set from one of a disciplined warrior to to a naturalistic border guard
- Find a way to give the fighter spells
- Grant a method for the fighter to wear light armor effectively in heavy combat
- Find a way to give the fighter nature/animal abilities
- Balance these features along with the other "fighter archetypes" and other class features.
A lot of work for a core base class. It would probably have to involve a lot of substitutions/replacements/kits, forcing the use of feats, and probably multiclassing into a cleric or druid.
Much easier to actually make a separate ranger class. And a separate barbarian and warlord class. And a separate fighter class and give this class more clarity and it own niche(s).
Now it woouldn't be impossible to give a fighter a "woodsman", "berserker", and "commander" build but the varrior class might have too much baggage at this point to do them justice without making it overly complicated.