My guess is a very narrow focus on optimizing certain metrics and a mindset that encourages "growth hacking" and any other means to reach those goals, even at the cost of everything else. *

They even described the design process that lead to some of their terrible UI elements in their tech blog: https://news.ycombinator.com/item?id=16226029

(* which, I suspect, is the kind of mindset spammers need to rationalize their actions.)