The issue is doing DLC for an open world game is hard. The way it's been done in the past is broadly one of the following:

• add a new zone that doesn't interact with the rest of the world
• add a new location, a few new maps that link to the original zone and some quests The issue is that that's not enough to necessarily make an entirely new playthrough worthwhile, but also an existing near-end save might trivialise loot and content.

The solution is so some combination of the following:

• Make the content spread throughout the world
• Balance the game so that new gear are choices rather than straight upgrades.
• Add new systems to engage with.

Fundamentally Bethesda as discounted the latter. It's done with classes, it's not added races, or new systems or new skills in years. They can't add content throughout, that would require creating the space for the content to exist in ahead of time.

Not that it can't be done, but that they don't have the future awareness to make room for it.