Using the iPhone in landscape means a whole lot of vertical scrolling, more than other phones because they only increased the height and not the width of the iPhone.
Keyboards like Swiftkey and Swype are far faster and more efficient than the stock Android or iPhone keyboards (portrait or landscape).
Swype and Swiftkey don't come default on every Android device.
And you don't have to exclusively read things in landscape. Just rotate back to portrait when you need to scroll that much.
How are you comparing the iPhone 5 in landscape mode to an Android phone in portrait mode? That doesn't make any sense.
You also have to scroll left and right on an iPhone 5 if you pinch-to-zoom; same thing applies to the S3. They both can do fullscreen in landscape mode...
I'm comparing the iPhone 5 to an Android phone in portrait because people keep saying the iPhone 5 lacks "width", but that's not exactly the case. If you want "width" on any phone, be it Android or iPhone, you only have to rotate the phone to landscape.
Some websites don't allow any zooming at all... whether on Android or iPhone. You can force zooming, but in such cases, both devices will benefit more from landscape than portrait.
Also just to make a point, there are actually fundamental differences between the way iOS renders websites as opposed to Android.
Even in portrait mode, some websites are more legible on iPhone than on any Android device. Here, take this example for the iPhone 5 vs Galaxy S3:
First, S3 portrait:
The texts in the first paragraph are barely legible. I have to zoom in to read them comfortably on my Galaxy S3.
Now, iPhone 5 portrait:
See how texts are much bigger in the same paragraph? It makes the paragraph slightly longer, but it's very legible even if I don't zoom in.
And in landscape? The difference really shows...
Galaxy S3:
And iPhone 5:
Texts are bigger on the iPhone 5 again. As Red Storm said, this requires more scrolling, but I can easily read texts on the iPhone 5 when I hold the phone further away. The Galaxy S3, on the other hand, would require me to zoom in a little bit before my eyes are comfortable with the text size, so the extra screen real estate is actually for nothing at all, and I end up having to scroll an equal amount on both phones due to this.
Also just to say, iPhone 5 in landscape is easier to hold and operate with one hand than Galaxy S3 in portrait since the device is physically smaller and significantly lighter.
Some people prefer the smaller texts on the Galaxy S3, I must admit, since it allows them to fit more on the screen, as you can see. But I don't. I prefer legibility of a website over everything else. And the less I have to zoom/scroll, the better. I end up having to scroll less on iPhone because of the way texts are rendered.
Now, before anyone suggests it: yes, I tried to increase font size on Android. It breaks the layout of many websites, and in general, it just doesn't feel the same way Apple does it. Some websites genuinely optimize themselves to be viewed on an iPhone, and so they always work better in Mobile Safari. Just like how many online video services serve direct MP4 containers to iPhone while leaving just the Flash container for Android. Favoritism is strong here, but eh... what can you do?
Edit: here's a screenshot to demonstrate what happens when I increase font size on Android:
See how those texts completely obscure the photos in the headline? It's more legible, but... it really breaks some websites almost completely.