That's still not when it first showed up, that's when they first identified it.

The first case was likely 1920, maybe even the tail end of the 1800s.

And back then, people just died from all sorts of shit and modern medicine was practically non-existent in the area. Even when it spread to more developed countries, it took a while to put the pieces together because it wasn't like aids directly kills. The aids just made all types of shit more likely to happen and kill you. That's why it felt like it spread so fast all over the planet. It was already there and people had it, they just weren't symptomatic yet and wouldn't be for months/years.

I mean, look at the nightmare syphilis was for centuries. If AIDS had spread a century earlier no one would have known what the fuck was going on. It was bad, but it could have been so much worse.