Components aren’t reliable. The whole thing might be duct tape and popsicle sticks. But the trick for SRE work is to create stability from unreliable components by isolating and routing around failures.
It’s part of what made chaos engineering so effective. From randomly slowing down disk/network speed to unplugging server racks to making entire datacenters go dark - you intentionally introduce all sorts of crazy failure modes to intentionally break things and make sure the system remains metastable.