The three games with most in common to Fallout 4 - Destiny, Borderlands, and Minecraft - all have unique quirks to them to make sure graphics aren't an issue. Minecraft's graphics are legendarily simple, Destiny splits its open-world into a lot of tiny, Crysis or Half-Life style little worlds, and Borderlands is cel-shaded to avoid the need for detailed textures. Fallout 4 tries and fails to emulate the other AAA open-world games rather than just either sticking to what they know or coming up with a new technique.
Open-worlds with a lot of depth in physics and several spaces to explore typically should not focus too much on graphics. The Witcher series was a result of continuous work on a specialised, in-house engine developed over years, so it's rather the exception.
STALKER and Dead Island are pretty old and dated, and Far Cry games and Dying Light requires fairly high system requirements. MGS and Dark Souls don't really have high-detail textures and meshes, but they tend to balance the overall look so well that their games usually tend to look good without needing them.
Other popular open-world games like Shadow of Mordor, the Arkham series, GTA, and Just Cause don't really have a high number of connected interiors, nor do they focus on physics too much and they are not in first-person, and as such they can focus much more into making graphics look photorealistic.
Simply put, Fallout 4 is like a jogger attempting to pull off running a marathon without a training plan, or any form of pacing during the run. It's attempting to reach for a goal they are simply not prepared to achieve, and something they just cannot catch up on.
Either they need to start ditching the "can pick everything up" feature (because it's a huge impact on performance), stop using the consistency by grid (where you drop a can and it stays in place for an in-game week) because it's also a huge impact on performance, segment the open-world (which wouldn't have been a problem if they used the classic travel system), or just stop focusing on photorealistic graphics entirely, and leave it to mods for high-end PCs only.