Complex software has complex failure modes though.
An application running on a single platform, self-contained and with some basic failovers such as redundancy (2+ machines running the same application), etc. should have ridiculous uptimes.
A distributed and complex system with interdependent components, under variable load, with different capacities for subsystems running across some thousands of machines will, inevitably, encounter some unforeseen state which degrades the system as a whole. It can be a small component that cascades a failure mode in an unexpected way or can be a big component failing spectacularly due to a bug, or race condition, or a multitude of other issues that are not entirely predictable and guarded against at the time of writing such software.
The latter is what has "wear and tear", it's not one software, it's a whole system of software communicating with each other in varying states of hardware decay, you can design and build it to be resilient against a multitude of predictable issues but you can never expect that it will run perfectly fine unattended.
