>>3242777It's called full frame because it covers a full 35mm film frame, it's as simple as that. Medium and large format are much rarely and more expensive when it comes to digital (and may not have even existed when the term was coined) and even with film it wasn't super common, where as everyone had experience with 35mm film.
What you're saying about focal lengths is irrelevant but also wrong, it doesn't change with sensor size. A 50mm lens is 50mm whether it's on full frame, APS-C, a 1 inch sensor, or even a medium format body.
What does change is the field of view, and as mentioned above many people have experience with 35mm film but not the smaller formats so the lenses on smaller format digital cameras are sometimes given their 35mm field of view equivalents. Also with FF being the largest commonly available sensor size it makes sense to use that as the standard for comparison.
