The reason why they're all similar is because they're easy. It's easy to find a center point of an image and it's easy to work 100% around that center point. Even if they don't realize, everyone who makes a header like this is using the same technique of finding two focal points, one of the image and one of the text, and lining the two up to try to show off both.
The reason why there aren't many headers that are different is because they're far harder, more complicated, and easier to mess up than something that just looks right. It's done again and again because it's satisfying on the eyes. Any thread I make in the future will probably have a similar formatted page header, because that's what you want in a page header: a visual introduction that doesn't pull your eyes all over the place.
The reason why threads have used this format again and again and again in the headers is because it works.
Following your style, here's another way a header could be made:
The problem with this is that your eyes are naturally going to want to look at both the text and the visual, but that splits the view. Something like this is fine farther down in a thread, because once someone is reading a thread you want them to stay longer, but as an introduction, when paired with the OP's avatar or something just to the side of it, you don't want someone's eyes to get crossed at the very start.
Another reason why this sort of thing happens a lot is that legs are
very awkward things, but especially in imagery. Between either them looking out of place in a composition or the source image that a header uses is cropped, often times people are going to want to cut that out. So, this happens.
Here, you can see that the soldier's legs are blurred out, which is extremely common, and it allows a clean look. Alternatively, if you're using a bolder foreground, like in the 28th or 66th headers, you can get away with having a busier background, especially if that source image was extremely difficult to crop. While that foreground/background look can be desirable, it often won't work for historical regiments that don't have a lot of good cinematic images of them, or for source images that are pulled from the Osprey illustrations. Here's an example, the header I made for the 92nd back in 2015.
In this, I used a fantastic plate of the 92nd that I found in one of the Osprey books. Since it was such a good illustration for what I wanted to accomplish, I decided to use the entire thing, as it was, as one continuous header. However, like most Osprey illustrations, the background was pretty plain, and would look bad against the whiteish background of the website. So, I cropped the whole thing. In this case, legs instantly became an issue because I didn't have enough space to include the entire illustration, and anything below the knee isn't that important to show in a header like this, in my book. The reason I went for this fade-crop on the bottom of the cropped source image was to keep a clean text, however, if I was to do it again I would probably do it a bit differently, adding some sort of far-background and a more pronounced text. The fonts in this header were the major drawbacks in my personal hindsight.
I also wanted to share this picture to prove a point I made earlier. Focal points matter so much, and that's why just about every decent thread ever that had a header centered that header. Looking at the 92nd header, does it look off at all? It should, if it's not fitting center of the page or your screen. That's why most people like to center their thread headers: it achieves the same goal as the headers shown in this thread, and helps to accent that goal really well. So, people do it, and will continue to, because it works; it draws the viewer's eyes into one place. In the case of the 92nd header above, that place is the horizontal line between the text and the soldiers, but thanks to the shape of the source image bringing the officers up front forward and the soldiers and musician in the back away, the primary focal point is still in that very center. The Gordon Highlander's emblem above it was controversial between me and Zzehth, who also worked on the 92nd thread, but with or without it, the viewer's eyes will eventually be brought in to that central focal point.
The original concept of that header was likely inspired by this header below, which was made for my regiment, the 91st, by someone, but I forgot who, it just wasn't me. Very talented guy, I think he made the intro for the 92nd (which was perfect, just poorly used on our part), so thanks a lot to whoever that was. I'm sharing this for two reasons: 1, whoever made that 93rd header clearly bummed from this one (lol), and 2, it's a great example of what centered focal points do for a thread.
In this case, the focal point may be hard to spot, but it's there. There's actually two: the officer in the center, and "91st." Because of the height and size difference between the officer and the soldiers, and the "91st" and the other text, the focal point objective is achieved.
And with that, tl;dr: headers follow this common pattern because it works really well for what it's meant to do.