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50mm gives the same horizontal FOV on landscape orientation as 35mm on portrait orientation. A compromise between the two is 43mm.

Within the so-called FF format, 43 mm is the diagonal √(24²+36²) = 43.2666 mm, therefore someone decided some day to call 43 mm lenses "normal lenses" as they are meant to be the as close as possible to a "natural human view". Which is broader than higher, so rectangular proportions appear closer to our human elliptical field of view habits.

With a doubtful eye on the question wether 7 mm difference in FL do matter, I think, no one ever evaluated a statistical examination like "what is your normal field of view" as I can't borrow your eyes to have a comparison between our most likely different fields.

It would never occur to me to call a normal lens as a lens which in portrait mode gives me the field of view of "the other" normal lens in landscape orientation. Feel free to do so, but in my cosmos is only a small room for (and a small number of pictures made with) "normal" lenses, I find them more dull than useful.

Anyway, all three together cost just as much as a better nifty fifty, so each Sony shooter can take out of it the most normal lens. Or just enjoy the wide angle with 1:2 scale.

The diagonal gives the normal for... the diagonal. A compromise between vertical and horizontal shooting. We tend to interpret what we see with the width. 50mm on FF gives a similar feel/FOV/depth perception as the central part of our vision (when you shoot landscape orientation). 35mm gives the same feel/FOV/depth perception when shooting in portrait orientation.
 43mm just sits inbetween.

Try calculating the diagonal from 36x36mm, or 24x24mm. You will start to understand why all camera makers used to offer 35mm and 50mm lenses as standard, and why you mostly found 43mm on cameras with fixed lenses.