Because we see cases like these all the time.
If "doing everything" they can do is talk on-line then what difference is it going to make? It doesn't even sound like either of these two can really afford such a long distance relationship. What are they gonna do? Just move in together? And who's going to be the one to move?
And yes, you can get tired, bored or frustrated with your partner in real life, but honestly, you're a hell of a lot more likely to grow bored of the one dimensional aspect of a on-line or phone relationship.
You can talk, and all you can do is talk.
You can't go for a walk, can't hug them, can't kiss them. You can't visit their place for an hour or so and head back to your house. You can't meet up with them real quick for some coffee before you start your day. You can't wake up with them, you can't make them breakfast. You can't see how they are with other people, experience things with them that neither of you have experienced. Can't take trips, can't visit each other's family. Cook dinner together or have sex.
You have less things to argue about because you're more likely to be on your best behavior when you're talking to them. You never have to work through real world problems together. Can't deal with a flat tire, or death of a loved one.
Even in a progressing LDR where the couples are actually taking concrete steps to moving closer, if not in with each other, many of these things are missing. Relationships can be difficult enough as it is. Having to deal with distance, lack of intimacy, finances, culture, family/friend approval are only strains on the relationship. You can be bullish about it now, but unless you're taking steps, somebody will eventually back down. Doesn't mean they didn't have their heart in it, it's just a fact, everyone has a breaking point.