Originally it was to save memory. Don't render what you can't normally see because it takes up processing power. But with today's engines and average computer specs there really isn't any good reason why more developers shouldn't be doing more immersive 1st person views.
That's part of it but it's mostly just that having a full first person character in the main environment layer is seen as kind of a cheeky move, and doesn't always work out so great because it introduces all kinds of camera clipping issues as well as the character either clipping through the walls or having an ultra bulky collision boundary to prevent that. Most developers just skip over it because it's just not worth the hassle, but for an immersive game like Fallout, Beth really has no excuse.