It's mostly because there is a demand for HA which requires multiple replicas. The moment you start going down that route you increase complexity. Whether most

It's mostly because there is a demand for HA which requires multiple replicas. The moment you start going down that route you increase complexity. Whether most things actually require HA is debatable, but a lot of businesses make it a core requirement and so it gets baked into the architecture. Personally I feel like most stuff would be better suited to having fast fail-over and recovery early on, but my advice rarely gets taken. Instead you end up with complicated HA architectures that nobody totally understands, which then (inevitable still) fall over and take hours to recover.