As others said, it isn't about scale, but reliability, and even things like patching. With a load balancer, you can easily patch your web servers, but very oft

As others said, it isn't about scale, but reliability, and even things like patching. With a load balancer, you can easily patch your web servers, but very often, DB servers go unpatched and un-upgraded for years since they aren't in a self-healing cluster. To add some detail, I worked on an e-commerce system that was like this. We would've lost something like $60k an hour if we rebooted the database server (yes, one server, nobody expected this sort of growth). So we didn't touch it for 3 years until it was about to run out of disk space. Then we had no choice, but then we set it up as a fully replicated system with a ton of disk space, making future patching easier.