I'm not buying the notion that Bethesda svcking at stealth is intentional. That's ridiculous, but I see what you're going for -- Fallout is not a racing game, so we are intentionally not going to feature operable cars, realistic driving physics, plush automobile interiors, racetracks, etc. I get that.
But.... while Fallout isn't a hardcoe stealth game, stealth is an undeniable playstyle choice for many many players. And if you're going to make it a viable option, you better develop the best [censored] system you can. You're proposing they do the opposite. Fallout isn't a hardcoe "first person shooter", should they then strive for terrible weapon mechanics?
The more likely reason stealth and AI are so lacking, is, well.... it's [censored] difficult to code. Just look at Apple and Siri, a 10-second conversation with her, and I want to throw my phone out the [censored] window. Apple has more than $2,000,000,000 cash on hand and even they can't do AI effectively. Anyway, AI is tricky. Couple that with uncapable developer expertise limited developmental resources and a sub-optimal "all-in-one" framework and game engine, you're going to get Fallout-esque AI.
Edit: words