This is because, with a few notable exceptions related to applications in scientific and mathematical number-crunching (like computer modelling and rocket science), when a computer system does work, we usually stop calling it a computer. When small numerical calculating computers became reliable and cheap and mass-produced, we stopped calling them "computers" and started calling them "pocket calculators". When personal computers became mainstream and stopped being niche toys for geeks, we started calling them "PCs" or "Macs", without really caring what "PeeCee" stood for.
In offices where the IT system works well, people tend to refer to the things on their desks as "workstations" or "terminals". These are things that you just switch on and start working at. They're functioning business tools.
So, once all the general-purpose systems that work are taken out of the equation, what we're left with is the bespoke systems, and all the systems that don't quite work, aren't quite finished yet, or need a lot of technical support and hand-holding. These are the scary, technical, sometimes-malfunctioning things that we still refer to as computers.
It's interesting to watch this change in naming happening with products as a market sector becomes mature. Home weather monitoring systems drifted from being marketed as "weather computers" to "weather stations", and in-car navigation systems shifted from initially being referred to reverentially as "in-car computers" to being casually referred to as "GPSes". Once the novelty factor has worn off, and people know that a product is reliable, useful and worthwhile, the "computer" tag gets dropped.
So as far as retail products are concerned, "computers", almost by definition, are the remaining gadgets that are either too new to be judged yet, or that don't work properly without a certain amount of expert hand-holding.
This also gives us a handy way of quickly assessing how good a company's IT infrastructure is. If you're visiting an office, and the general office staff refer to their "computers", then the chances are that either the staff aren't very computer-literate, or the office has just been undergoing a painful IT transition, or ... their IT systems simply suck. Try it.