You're right. That's not the sole reason. The primary reason is that originality is a relatively high bar -- particularly for the Internet.

Just look at Twitter. So many tweets are retweets of what someone else said.

That's not the point. I am not talking about lack of originality. I am talking about failure to notice duplication in short list of supposedly interesting content published by oneself.

The reason is plain and simple: when you build a list for an audience, it needs to have many bullet points. So you are tempted to use the same fact twice with a slightly different take.

"2 things you didn't know about entrepreneurs" sounds less interesting/professional/worth reading that "10 things".