It is strange isn’t it

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It happened as 35mm SLRs with changeable lenses became popular amongst amateur photographers. There was the 'standard' lens the camera came with, usually a 50mm (it was fairly unusual for all but pro-type 35mm SLRs to be bought 'body only' between the 60s and early 80s) then, in the earlyish days, you'd have a wide angle (often a 35mm) and probably a telephoto (usually a 135mm - which was regarded as a telephoto lens in those days). That would have been a fairly comprehensive set for an amateur between the 60s and late 70s, with maybe a 90mm instead of the 135mm if they were into portrait photography.

Due to that, 35mm photographers got to know what the framing looked like through each of those lenses and also got used to those focal length names. So when digital came along with different sensor sizes people wanted a reference point they understood; and that system remains in use today, and will probably continue to stay in use due to the rise in the popularity and affordability of full frame sensor cameras (both SLR and mirrorless).

I'd agree with Garry's point above, I haven't a clue what the actual mm zoom range was on the Canon EF-S lens I had for the crop sensor DSLR I used to own, but now I've switched back to full-frame I know I have a number of lenses (zoom and fixed focal length) that cover the range from 17mm to 300mm, and can remember the focal length range of each of the zooms. No need for me to think of 35mm equivalents any more, I'm back to what I know! :)