This is a fairly frequent problem with the 11sp shifters from 6800 onwards because the cable makes two 90 degree bends in a short distance at the lever.
The older design did not bend the cable so agressively and cables lasted longer (though they were more painful to replace).
To give some context, I service a rider's bike where R8000 levers are used. As he usually rides 400-500 miles a week depending on training schedule, he will see broken cables a few times a year, even where the best cables are used.
In more anecdotal evidence, I provide on-course mechanical assistance at a large annual sportive event. Last year it was run, I had to do three roadside gear cable changes in Ultegra shifters out of a field of just over 1000 riders. These bikes were well kept and serviced, from various parts of the country. If we extrapolate that not every rider has ultegra and not every ultegra rider gets a broken cable on a 130-mile ride, it's a high failure rate.
Regarding shifting, in a hilly area, the rider may shift rear gears 1000 times or more when riding for an hour, so the cable does alot of work! (data from Di2 connected Garmin computer, various riders).
Ultimately the "solution" will be an updated lever design from Shimano or to replace inner cables pre-emptively at each 1000-2000 mile service, only replacing the outers at a full service.