I'm quite sure liars do exist.
I'm also struggling to see how some of the posts here can be true, under the current benefits system.
GPs don't get to sign people off for ESA for "years and years", even if they wanted to.
The only post that comes close to explaining how the OP's story could be true (as opposed to a slightly mistaken joining of dots from different eras), is SoonToBeSix's, which claims some people still haven't been ported to ESA.
And that's also demanding quite a lot in the belief stakes, to say that over 6 years after ESA was brought in, there are still people who haven't yet been processed even once (for scale, I've been through reassessment 2 or 3 times since ESA was brought in).
I quite agree with the OP that living with people with any chronic illness can be frustrating, and mental illness and addiction particularly so.
And I can completely see how an addict would attempt to reframe their issues into something they themselves find more acceptable.
I also agree that some people in the world are lazy.
But I'm not prepared to sit by and let anyone perpetrate the much-loved myth that under the current system, large numbers of perfectly healthy people can wander into their GP's, tell a sob story and get signed off on incapacity benefits by the GP for years and years. There will always be a few try it on, but anyone who does will have to put in a fair bit of welly to the act and will still end up being turned down by the DWP at some point.