In some ways it's cultural, but in many ways it's political. That sort of overhaul would have to come from the top. No-one wants to be the person who suggests it since, if any income drops as a result you'd be in a hard position. So, many lower sections jostle for space and they all get in. (I heard this from a Rakuten frontend performance engineer. Other companies may vary.)

Edit: it's also extremely unlikely for a designer with a strong vision and skills to rise in a large company. Most will turn freelance long before they get there. Most upper management seems to come from eigyo - sales. I don't think engineers (basically just troops to implement the sales guy's vision and take the blame when something goes wrong) are in a position to easily climb the ranks either.

Edit2: I don't think the feature phones argument still holds water. Most sites will redirect for feature phones on the user agent level - they have too. Many models and carriers have their own quirks (eg only tables allowed, only inline styles allowed, different emoji codes) around which a large but dwindling infrastructure exists, catering for normalizing across models using template generated html. The browser versions are separate.