Failure is such a subjective term, and defining it yourself in such an over-arching rule is in and of itself highly suspect towards clear biases. I am more interested, really, what success means if failure carries such airs around it in your mind. Not to mention the fact that majority of the problems presented affect all roleplays. I have never really experienced a roleplay that "finished," per example, and from my experience the same applies for a very vast majority of people. Roleplays aren't doomed to fail for any real reason, it is the participants (gm-included) that make any story or setting fail eventually, or miraculously finish before it does. 

Mechanics and stats can frankly become incredibly boring, and they aren't exactly as fair as you want them to seem, since you could get bad rolls, get forced into encounters you don't really want to be forced into, and the general "game" mentality itself lends itself towards powergaming and min-maxing which isn't something that should exist in roleplaying in my opinion.

A lot of people disagree, and let them I guess, as there are plenty of "nation games" on the internet and the like, but that seems like such a bore, annoyance, and needlessly brainless experience in my eyes. I write for story, I don't write to win, and if you have another perspective I dare even say it is an unhealthy approach when RPing at all.

I could title this thing "why all roleplays are doomed to fail" and I could pretty much argue much of the same points. It is simply a matter of individuality and personal circumstance that roleplays die off, people lose interest, and maybe even over-excert their enthusiasm. Things could turn up in their lives, forcing them to deal with it, and potentially forget about the roleplay or lose interest. It is a common thing and a general understanding amongst the community of roleplaying at large. 

I am in fact mildly surprised someone thinks like you do, no will intended, as I have never really met people like you who seem to prefer treating stories like games where winning is the most important thing.