They have. Read the info, Eder is given as the example. You also can't choose a subclass, they are preset. Eder is a vanilla fighter, a vanilla rogue, or a vanilla fighter/vanilla rogue. It used to be 3rd edition style multiclassing, where you can choose a second class (and subclass) on level-up. Thus the ability to choose a wacky second class was a product of the game system, not a design choice.
It now works something like DOS2: when you first meet Eder you have dialogue options: "I would like you to concentrate on being a fighter", "I would like you to concentrate on being sneaky", "I would like you to choose a balanced path". Clearly, there will not be a dialogue option for every class and subclass in the game.
Be careful when making such statements, provide a link to your information or please stop spreading misinformation. I have read all info available several times and there is to my knowledge no indication anywhere that companions for certain are locked to vanilla classes and can't multiclass outside the two preset classes. To me, that just seems like your interpretation. If I'm wrong, then I'm sorry.
My interpretation, and I think many others, is that when you first encounter a companion you are prompted to choose one of two classes (Fighter and Rogue in the example of Edér) and an optional subclass for that class. In that window or a second one that pops up after you will be given the option to multiclass that companion into any other class (except the priest/paladin restrictions) including a subclass for that second class. Does not have to be a dialogue option for every class and subclass, just something similar to character creation. However, nobody can tell for certain how it's going to work, at least at this point.
From a design point-of-view it doesn't really make sense to give the companions too many restrictions when there is so much work put into their dialogue and story, since if you want many multiclasssed characters you could circumvent such a restriction by hiring adventurers from an inn.
Would be really cool to get a clarification from a dev on this matter