For example, someone wants to make the archetypal knight, what subclass should they take? It doesn't matter. It's not clear. Their choice isn't any easier. When someone makes a wizard, what they want to do in the game leads them to their subclass choice. When someone says "I wanna blow things up" then you point them to evocation, but if they want to be more tricky you go to illusion or enchantment. No detailed reading of the subclass is needed. When someone makes a fighter, the choice of "I want to be a great knight" or "I want to use a spear like the Oberyn Martell" or "I want to take the hits like a boss" then you need to read and re-read the options.
Similarly, reflavouring only goes so far. Someone wants to make a Madmartigan or Indigo Montoya and be great with a sword. Saying "great, just pretend you're the best swordsman in the land" isn't as satisfying as having something, anything to make that actually true.
When you're walking a new player through character creation, with every other subclass you ask "what do you want to do?" With the fighter you don't ask that but instead ask "how do you want to do what you want to do?" Which isn't very easy or intuitive...
You are seeing this as a bug, and others are seeing it as a feature. When you want to make the archetypal knight, you take the fighter. Whether it is a Champion Fighter or a Battlemaster Fighter is more about how you like to play than it is about how you view your character. (Though, as I said earlier, I do think there are some flavor implications with either subclass.) Is your problem that beginning players will have a hard time coming up with a
character (as opposed to a statblock), or is it that beginning players will have a hard time mapping their image of a character onto a particular build? For the former problem, yes, the fighter, in its generality, doesn't necessarily tell new players, "This is exactly what a fighter is," in the way some other class builds do. On the other hand, I would argue, the fighter is the easiest flavor to come up with on your own. Most players, even inexperienced players, can come up with some flavor concept for a fighter. I don't really see the latter as much of a problem — again, in this case, either subclass can work for most fighter concepts. It's all about playstyle.
Not to mention one of the most important features of 5e, which is that the fighter's subclass
doesn't have to be chosen during character creation. Your new player doesn't have to worry about which subclass will fit their archetype, because they get to spend a few sessions becoming familiar with the fighter and the character before they have to make a choice. And then, when they do make a choice, the contrast is super stark and easy. Do you want your character to do more than it already does? Choose a battlemaster. Or, if you want magic, choose an eldritch knight. Are you happy with "I hit it with my axe?" Choose the Champion. It will never get any more complicated. You will never pick up another new ability. Your combat mechanics will always be based around "I attack." This decision doesn't have to inform the character concept, because, by now, the player has already had several sessions to flesh out a concept. It's all about how that player likes to play.
Most of the flavorful classes, by contrast, force you to make some sort of subclass choice out of the gate. Sorcerer's and clerics choose subclass at level 1. Warlocks effectively have 2 levels of subclassing — which kind of being their pact is with, and then, at level 3, their pact subclass. For some people, that is not a good thing! That means, before you've ever played a single game of D&D, you have to pore over several different build options and choose between them. And now, your build option comes with a ton of baked in flavor that kind of dictates not only a lot about your character, but also how your DM has to build the world around that character. A woman in our group played a Great Old Ones Warlock, because she loved the telepathy and the "cosmic" quality, but didn't actually care for the whole Lovecraftian mythos stuff. (She'd read quite a bit of Lovecraft and found him distasteful.) The DM had one idea of what her character's backstory was about. The other Lovecraft-obsessed players had another idea (3 of us met playing Call of Cthulhu), and she had her own idea. Trying to reconcile all of it was actually pretty frustrating for her and the DM, and while we ultimately found a backstory that made sense for her and the DM's world, it left a bitter taste in her mouth. Bummer for a new player!
As a fighter, on the other hand, she could have chosen almost anything she wanted, and not have to worry about stripping out a bunch of flavor assumptions built into the character. Then, when we reached level 3, she could have said, "ok, all this 'I attack' is getting boring, give me some of these moves," or she could have said, "I'm more interested in the role-play and thinking outside the box than I am in the tactical combat. Let me be a champion."