How common is it to buy one extremely powerful database server? Wouldn't you need the high availability properties of something like a Galera cluster? Or are most people accepting the single point of failure at huge database server?
While you do want HA you also want to keep things simple. You get great HA with just one backup machine. The times you lose two machines are so few that you might crash the site with software failures far more often anyway.
Databases have inherent locking problems that takes more resources to resolve with more machines, more so in some database systems than others. When scaling you often hit a point where you get down to macro logistics, so imagine a car highway: You can get more throughput by adding more lanes. But what that all cars are going to merge to one lane at some point in the middle of the trip (because of a tunnel, bridge or something: it's impossible to have more than one lane at that point)? Now you won't get any throughput benefits of the multiple lanes after all! Just more latency because you get queues up until the single lane and because of the queues all cars are going slow and need to accelerate on the single lane, so the average speed is low too. You are also having too much resources after the single lane because you can never fill all those lanes. It might be better to have one beefed-up single-lane road all the way that people can go fast on. Basically: Remove locks and you get better overall performance.
Yes, this is on the expense of HA. Yes, the costs of scaling up grows asymptotically faster than the costs of scaling out. So this is definitely a trade-off in some sense.
They just run everything through one live database all the time under normal conditions. Personally, I think this is much simpler to manage than to be regularly spreading out queries over many queries.
(I honestly don't know, it's just surprising.)