To answer your question about why most people counting calories fail, the answer is pretty simple. They're either unable to stick with it for whatever reason, or they're wrong about their calorie intake (either by improper measuring, underestimating, forgetting stuff, or, yes, sometimes lying to themselves). It drives me nuts when people quote the statistics about weight regain after calorie counting as evidence that it doesn't work. Well, yeah, of course they'll regain once they stop controlling their intake. That's kind of how it works, and it doesn't mean that the problem is with the calorie counting.
And as for marathon runners gaining weight on 1200 calories a day? Sorry to be confrontational, but I have to call bull****. That's not physically possible. Weight loss/gain isn't magic. You can't put fat on out of thin air - it's made out of something, and that something is calories.
(just as a note, I'm not saying that calorie counting is the ONLY way that works, although it is for me. Any method that results in eating fewer calories that you burn will work, whether that's by counting, cutting out food groups a la keto, portion control, whatever)
Former maintainer, now losing again in 2014 after my second baby.