There is another serious point about this - state support for SSP has been withdrawn so this is a cost to business with very little recourse - effectively you have to try and say that the person is no longer capable of doing the job and this isn't always easy - or comfortable - as most employers try and do right by their staff.
The problem isn't really the doctors - who I'm sure have a nightmare job assessing who is or isn't - they don't have the time to do full assessments, and I wouldn't be surprised if there is an element of being on the safe side. There are also web sites where you can download sick notes - realistically how would an employer know?
I don't know what the answer is, but I doubt it's Capita