If I say imagine the person you think you'll end up with then the chances are you're going to choose someone who looks somewhat like you. This is natural as I just asked you the most logical conclusion to your love story. The thing with online dating is that you have so much power to choose rather than meeting up out of being in the same place at the same time. When you're talking to someone and they're talking to you it's because you both think the other is attractive and makes sense given what you want out of the app or site. This leads to a lot of preconceived notions about what someone will be like based on what they look like.
Too little commitment
Ghosting is a lot easier when you meet through the internet. You don't really feel like you owe this person anything. The truth is, unless someone harassed you then you owe it to them to at least say you're not interested.
When you text a lot and talk on the phone but don't meet up there's a lot of room for the imagination to take hold. The problem with this is that reality doesn't often live up to imagination. You can find yourself spending time with someone only to find out that they really aren't who you wanted them to be and while that's ok and they should stay themselves--you may not have as much in common with this new person.
Too many of the same problems
People who use dating sites I think it's safe to say do not have an overflowing dating life from people they meet in person. That's not to say all internet daters are unattractive and/or weird. For whatever reason, however, everyone on a dating app by definition has a problem which inhibits them from dating in person first. This again relates to point 1 about too specific, people don't meet randomly on a dating app that part of the game is an illusion. Even if two people meet up and then one day get married I'd still say that that's both of them settling by definition of the fact that they would have preferred to meet in person first.