He has sent many messengers and created a world filled with signs for people to reflect. Why would someone acknowledge His existence, but refuse to worship Him and to obey the commands He has told people to obey (or, the commands people think he has made known)?
What I understand less is refusing to acknowledge the presence of God. It's not just an argument from complexity, the males and females of all species are suited to go together. And even snails which reproduce sexually, despite being hermaphrodites, have a mechanism to cope with their natural tendency to both want to compete as the "male" in the relationship. A love dart so to speak. Okay, let's think about this, it just happened that their natural tendency to compete was compensated for? No, SOMEBODY knew this would be a problem, in fact designed both the problem and the solution.
This may be too risque for younger viewers.
Or how about this? You know why a meteor doesn't destroy the Earth like in all the big films (Deep Impact, Armageddon, probably a few others)? Well, we have Mars in the way. We have an asteroid belt in the way. A planet with rings in the way (these rings are made from mini asteroids themselves), and some of the other planets have mini-rings themselves. Oh yea, and several gas giants in the way. Gas giants act like a pillow, trapping solid objects in their gravity, slowing them down, and crushing then to dust. These things are all arranged.
It's not an argument from complexity, it's an argument from planning. We cannot take you seriously, and in fact will mock you if you dispute this. Either in denial, wilfully ignorant, or crazy. It's so extreme that someone would say that, so out of touch that Timothy actually called people who denied the existence of God "under a delusion." Yeah, you've heard the God Delusion book, well there's more of a case for the opposite. This things just don't work without a central planner.
But on the other hand, the idea that you have to WORSHIP such a deity is not a given. I don't expect anyone to do so. In fact, you know that "choose this day who you will serve" quote from the Bible? (even non-Christians know the song "You gotta serve someone" if they listen to blues and jazz) Well the entire passage goes that the Jews say that they will worship God, and the guy is like "No you cannot worship God" and gives loads of reasons like God not being cool if you say that today and suddenly bail later on.
Totally understand why people might understand there is a God but refuse to worship God. That's fine. But being in denial or worse, trying to talk others out of their beliefs, nah that's messed up. Especially since they tend to try to bully people new to their faith.