Many relationships don't work, in person or long distance, and there are many, many reasons for this.  Long distance relationships have many disadvantages from the start that in person ones don't.

The true physical attractiveness  of both people is unknown to the other and this is generally one of the first things people find out in person.  Now, looks aren't everything, and in a perfect world would count for very little, but it is somewhat important for intimacy.

The true person does not always show through in long distance communications.   Many people use the anonymity of the internet to hide the things that keep them from forming in person relationships.  They have more control over what they reveal and conceal, and have the ability to lie without being discovered. When the reality is revealed, the relationship is shown to be based on misinformation, and can come to an abrupt end.

Long distance relationships do sometimes work out.  I have seen some examples.  On the whole, it takes two honest people for it to work.