Because these are not a bunch of amateurs working in their basement. They have a full art and design team. If the problem really WAS that their artists and designers sucked with hairstyles, then they'd either ensure proper training or hire people with actual skills in the field... and they'd certainly NOT keep people who are bad at creating hairstyles working on that feature. That's seriously basic management here: you assign the right people to the right tasks, and with a AAA budget they certainly ARE capable of hiring accordingly.
The other alternative is that the devs don't even recognize that the hairstyles are rather lackluster, or that it's been a problem in past games... which is just about as ludicrous as saying they'd keep artists/modelers who are bad at hairstyles working on hairstyles. For one thing, they're quite aware of the complaints people have had with past games.
I'm not sure if you really understood my question. While on the surface the idea "they are not a bunch of amateurs" etc may seem solid to you, did the hair their team(s) produced in the past games fill you with some actual confidence that they can indeed produce decent looking hairstyles? Have all their past designs left you with firm belief that all the decisions their teams make are good and without fault?
Hair *is* hard to do well. And having full art design team doesn't really guarantee that there's going to be someone in it who is good at this particular task, because game artists have to do *all* sort of stuff, not just hair. And so it's not as simple as just 'hiring someone with actual skills' or doing training. They already have tons of plates to spin and tight schedules to meet. And not only are people with skills good enough to do this stuff well rare, they are also sought after by many. They aren't just sitting there, twiddling thumbs and waiting for BioWare to give them a call. In such environment it'd be hardly "ludicrous" if BW couldn't spend enough time on patching this particular weakness (if they even recognize it) or were simply unable to find a person for such position that'd actually want to work for them. Less ludicrous than the "engine limitations", in any case.
So we're left with technical limitations... and saying that good hair has been done in other engines doesn't mean much, even good hair done in Frostbite. Because DAI involves several more challenges due to the customization of our characters (both in appearance, and in equipment).
Customization of characters and gear at best affects longer styles, if you care about clipping. It means nothing for short styles and quality of the textures. What technical excuse would you give for these?
Also, when these exact several other challenges (and more) were
quite successfully tackled by other games, what difference is there left to explain the disparity, other than the team which worked on it?
(that's not to say FFXIV has some exceptionally good hair, but when it comes to basic geometry, textures and variety provided? It seems miles ahead, and that's not only in a game with a number of customizable races and equipment, but a MMO to boot that has to work in extra harder conditions due to much larger numbers of players on screen)