Well one of the reasons without a doubt is lots of the staff are taking the p***,i know of at least 2 in engineering who are on " long term sick " running their businesses,last year during the summer BA were cancelling flights which we thought odd at the time - turns out 500 cabin crew phoned in sick during ascot.
They seems to be totally inept at addressing out of order sickies - personally a video camera would suffice - what's the problem? - if your sick and caught at say Ascot ( and there must be hordes of BA manager / Director types go there ) its " Gross misconduct " - Surely?