Howard seems to have acquired the habit of picking my brains about relationships lately. (Isn't it enough that I write in my blog, Howard?) Since Howard is also a techie, let me put it in techspeak: I don't have the bandwidth to provide a pull-content service; I only push content. But I'll answer your question this time, since I don't have material to write about, being loverless at the moment.
So, why doesn't romantic roleplay last? I'll start with the obvious answer: It's roleplay, for crying out loud! How many roleplays do you know last more than a year?!?
Okay, now the serious answers:
1. It's usually because of sheer incompatibility. If you were looking for a real life partner, you're more careful about whom you pick. Do you share the same interests? Do you subscribe to the same beliefs? Is their personality compatible with yours? Can you live with this person for a very long time? Will this person always have your interests at heart? Do you have similar dreams?
But in a roleplay, you only care about superficial things. Does the other person want to do the same roleplay? A Gorean will look for another Gorean; a furry will look for another furry. And, most importantly, -- and, yes, I'm saying this very bluntly -- is this person willing to have sex with you?
2. Roleplay is not authentic. You might be feeling cranky and crabby one day, but you have to be nice and sweet, because, well, that's your role. Crankiness and crabbiness is reality, and your SL lover isn't looking for reality in a roleplay. Your SL lover is looking for a fantasy, and so are you. You wouldn't want a cranky and crabby lover ruining YOUR fantasy either, would you?
3. It takes a lot of energy to maintain an illusion. It also takes a lot of energy to deal with the emotions that the roleplay generates. Eventually, one or both of the partners wonder if the roleplay is worth the energy they expend on it. After all, it's just roleplay.
4. In roleplay, both parties fall into their roles right away. Not only do they suspend disbelief, they also suspend mistrust. But the trust they have is fleeting and fragile, because it doesn't have the requisite foundation that is built only by getting to know a person over a period of time. Without that foundation, it is even easier for trust to turn into doubt. And how can you expect a romantic relationship to last without trust?
I'm sure I can think of other reasons why romantic roleplays don't last, but I've used up my available bandwidth. So any other reason I come up with will have to wait for the next CPU cycle.