Here's the problem with online relationships, you can pick them up and drop them whenever you like. You can do that in real life to a degree, but sometimes you are forced to confront friends in real life when you don't want to. A real friend requires surviving a degree of confrontation.
This is the crucial difference. The online abstraction facilitates behaviors that are destructive to friendship, like ghosting, blocking, or even banning if you have admin powers in their meeting place.
Also, since real life is abstracted online, small issues tend to become dramatic because. It takes courage, and some recklessness, to open yourself to a total stranger that might not even be what they claim to be. For us who saw the initial anonymous wild west of the Web, it's difficult to trust people online. It might be different with younger generations that grew up with having their real identity open in several mainstream places.