> a billion USD, Google could spend $100 worth of human curation per Wikipedia topic
$100 would buy how much? Probably 30 minutes of my time, if I'm feeling very generous. Actually, probably 10 mins if it's Google, because I'm not giving Google any discounts, if I'm to work for the Evil Empire, at least I want to get rich from it! That's not counting training costs, transaction costs, legal compliance and HR benefits costs, etc. etc. So, how much work I'd be able to do with one-time investment of 10 minutes? I don't think too much, even for the topic I'm an expert in. Maybe I'll be able to notice and fix one error, once.
And then, the data changes all the time. People do new things, people change jobs, people change names, people are born, people die. You have to run very fast to just stay in the same place. And then you have 200+ world languages (surprise, not everybody speaks English!) - Wikipedia actually has 300+ but let's drop the most exotic ones.
So a billion dollars wouldn't get you as much as you'd think. You probably need to bump it by couple of orders of magnitude. Which gives you an appreciation of how much value people are actually willing to donate completely free, if motivated correctly. Unfortunately, there's no way Google could have it - except through an intermediary like Wikipedia.