The EU IV team doesnt have the power of hindsight that all of us armchair developers have. Thats how that happens.
Someone thought this project would take x month. And then it took more month and other plans (new studio, vacation, not dying to the plague) started to interfere with production. Which slowed it down further.
And while "just delay it until its ready!" sounds like a sensible thing to say, Johan has posted multiple time, why that is just not a thing when you are not steam or Blizzard or GoG (guys with their own platform).
Or in other words: if you want to play paradox games bug "free", just dont buy them when they have to release them. Buy them, when you think they are ready.