Where does all this sulphuric acid come from? Does Earth have the same amount of sulphur, just locked up in rocks? If so, why doesn't it react and precipitate out on Venus?

Okay, time to read the Wikipedia article [1].

So, Venus's atmosphere actually has four times as much nitrogen as Earth's. But that's overwhelmed by having about a hojillion times more carbon dioxide. There's more than enough nitrogen for an Earth-like atmosphere, crushed down into a tiny space.

Plus, the lack of a magnetic field means the solar wind hits the atmosphere harder than it does at Earth; hard enough to blow water vapour off the planet, although i don't know if that's in significant quantities.

Apparently at the surface, the CO2 has enough density and pressure that it's supercritical. So it's not so much an atmosphere, as an ocean. Kind of. We don't have a word for what it is, but it's atrocious.

As for sulphur, it came from wherever (volcanic eruptions, let's say), but the reason it stays in the atmosphere is that sulphuric acid never reaches the ground: if it falls as rain, the heat of the deep atmosphere evaporates it before it lands. I assume there's still a lot of gas-phase sulphuric acid at the surface, but perhaps that doesn't allow a fast enough reaction to take it out of the atmosphere.

And finally, Earth had an atmosphere like Venus's during the Hadean era, from its formation to four billion years ago [2]. Despite being ludicrously hot, liquid water oceans were stable due to the high pressure. Those oceans apparently allow some chemistry that takes most of the CO2 out of the atmosphere. For some reason, that didn't happen (or stopped happening) on Venus. Maybe because the water was whipped away by the solar wind? Or because Venus was just a bit too hot for liquid water? Or because you need not only oceans but plate tectonics to bury the carbon deep in the crust, and Venus doesn't have plate tectonics?

Don't ask me why Venus doesn't have plate tectonics. Probably the same reason it doesn't have a magnetic field, which on Earth is generated by stuff convecting around in the core.

So, total basket case really.

[1] https://en.wikipedia.org/wiki/Atmosphere_of_Venus

[2] https://en.wikipedia.org/wiki/Hadean