The problem is sizing and consistency. When you're small, it's not cost effective to overprovision 2-3 big servers (for HA). And when you need to move fast (or things break), you can't wait a day for a dedicated server to come up, or worse, have your provider run out of capacity (or have to pick a different specced server) IME, having to go multi cloud/provider is a way worse problem to have.
Most industries are not bursty. Overprovision in not expensive for most businesses. You can handle 30000+ updates a second on a 15$ VPS. A multi node system tends to be less reliable and more failure points than a single box system. Failures rarely happen in isolation. You can do zero downtime deployment with a single machine if you need to.
Just like a lot of problems exists between keyboard and chair, a lot of problems exist between service A and service B.
The zero downtime deployment for my PHP site consisted of symlinking from one directory to another.