I can't speak for everyone, but I suspect that science acts as an authority and, even if our understanding of how evolution works is limited, we still accept it. We usually aren't scientists or biology graduates with a detailed knowledge to build on but accept what has been repeated to us by adults, school, the media, etc. To really engage with creationism means having to get past the trained reaction of thinking it is "absurd" or "nonsensical" based on the assumption it is impossible and couldn't ever happen that way. For example, it is hard to keep a straight face when confronted with the banana as an example of intelligent design even if your willing to concede he may have a point.
It is therefore easier to challenge the
authority of a creationist by arguing their belief is illegitimate, than to actually look at the arguments. This is particularly true if the debate gets reduced down to a "faith versus facts" situation as that usually ignores the middle ground between evolutionary and creation positions such as old-earth creation, natural theology and enlightenment deism where science and religion are- at least in theory- compatible. I think if you can get the discussion away from young-earth creationism over the account of creation in the book of genesis and have someone willing to go in that direction you are more likely to get somewhere because it won't rely on the authority of scripture as the word of god. To reach a common understanding, you need a common source for that understanding and atheists aren't going to accept the bible as evidence. It really depends on the methods you are using to decide how we "know" something and how we "know" the origin of species, nature and the universe more generally.
It's also worth keeping in mind that the majority of people who use the internet are
already convinced of their own views and the most vocal and active members are usually going to be the most rigid in their positions. It is not automatically the case, but if you are someone who is looking for the challenge of debating online you probably already feel very confident in what you are saying before hand and aren't likely to change your mind. Even if a debate can make a dent in a person's convictions, such as by presenting new arguments that they haven't heard before, you are still relying on people being intellectually (and emotionally) honest enough to concede that. It can take alot of time for someone to change their mind and online debates are generally a very small time frame or window in to what people are thinking and how it might develop in the long-run.