From any other company that would be a logical explanation, but in Apples case, the dongles are one of the greater sources of revenue.
Well, they did include it with the iPhone 7 (obviously realizing there would be too much pain if they left it out). So they may just be hedging their bets and trying not to alienate too many potential buyers. Generally, I disagree with the (very popular) sentiment that Apple is just trying to stiff customers on adapters. I think it's about simplicity and their minimal aesthetic. They decide on the simplest configuration, then let users tailor that by providing adapters. I don't like it, personally, but it's been very consistent for many years.