Back then it was probably a lot easier to make more models, and the quality would be better with a single shot. Some of the original footage has blocky areas where the merging wasn't perfect. Didn't matter back then, it was far better than most anything else, but three model shots plus a background would have been ugly. Original Star Trek footage also has some of that.

Or to put it another way, we couldn't have faked the Moon footage because the technology to fake it wasn't created yet. It was easier to just build a big rocket and go.

I wonder if it has to do with ease of compositing multiple models. If you have three X-Wing models in the same shot you can have one in focus, with the others a little blurry and if the front one passes in front of the others perhaps it makes the compositing easier since instead of cutting two models separately and trying to make them look correct as they pass in front of the camera, you just cut them out together.

Simply Red standing by