Because most stores don't sell it as "Headlight Fluid", and in fact they never really have. You must be experiencing a Mandela effect of sorts. To prevent confusion, all brands agreed a while ago to sell their product as "Blinker Fluid". Valvoline for a time sold their product as "Headlamp/Blinker Fluid" (see pic related), but that was years ago. There are also vintage bottles produced by the private firm "Wilson's" (bankrupt since 1976) that used to make separate blinker/headlight fluids, but that was from the time prior to when the formula was optimized for all three lamp types to operate on the same liquid. Much like unleaded in a classic car's engine or modern motor oil, they should happily accept modern blinker fluid.
You might be interested in Ducati's "natural mineral water", as that's a more generalized lighting fluid designed for motorcycles, albeit modern ones, though like the previous point, should work on vintage headlamps just fine. You may need to pre-mix it with some Elbow Grease first however.