Social norms
The math for handshakes is difficult. You have to stand, look, squeeze, time and end in just the right way or it's weird. Skip the handshake or do a six-second version and people look at you funny.
Interpersonal relations have had thousands of years to develop. Online, there's been no time.
There are people who tweet in a way that rubs you the wrong way. Marketers who build businesses that seem scammy to you, or build websites that feel wrong. I get plenty of email from people that just doesn't feel right, whether it's in ALL CAPS or just difficult in tone or approach.
How do norms get formed? I think it's simpler than it looks: we interact with people who use the norm we use. We follow or read or hang out with people who use the same social constructs we do.
There might be people at the party down the street who are quite comfortable with each other and the things they're doing with or to each other… but you'd hate it. So you don't go.
Cliques form, which become communities and then, eventually a norm arrives. People like us like people like us.
If you're not attracting the people you want to be attracting online, perhaps you're not acting the way they do.