Brake cables need periodical exchanging. Many people don't do this for some reason. A well maintained brake cable will support your whole weight, one that is ignored for a decade or so will snap when you need it most.
There is no periodical roadworthyness review for bicylces, if I look at the things people sometimes cycle on I shudder. Broken / not properly tensioned spokes, cracked rims, missing brake pads, brakes not working at all, fixies, broken ball bearings, skipping chains etc. Bikes need maintenance, like every other vehicle.
(a) it takes effort; (b) it’s working, right? so why would I spend $6 + my own labour, or probably $50 for someone else to do it, on a new cable?
You are quite right that bicycles need maintenance, but they work well enough for many people with limited to no maintenance, so that even when they experience what their bike is like when maintained, they often still won’t find it worth the bother. (I would be inclined to be like that if only using the bike for fairly short trips; as it stands, my standard trip is 40km each way, so such losses would become too galling.)
This preference not to maintain incidentally points to a way in which I think electric drive-trains have real potential: they should require much less maintenance, and generally retain higher efficiency. “5% less efficient than a chain”, perhaps, but that’s a maintained chain. Minimally-maintained chain drivetrains will probably have lost that 5%, and unmaintained ones can easily lose 20%.
An unmaintained chain can break, and that can cost you a few thousand in dental work or your life if you are extremely unlucky. Better keep it lubed and make sure the sprockets aren't worn too far (especially important on derailleur bikes).
There is no periodical roadworthyness review for bicylces, if I look at the things people sometimes cycle on I shudder. Broken / not properly tensioned spokes, cracked rims, missing brake pads, brakes not working at all, fixies, broken ball bearings, skipping chains etc. Bikes need maintenance, like every other vehicle.