I can't speak for anyone else, but for myself, I first started visiting religious web sites back when I was learning about different religious beliefs. I was interested in reconciling my own beliefs and figured exploring other belief systems would be a good way to do that. This included visiting this forum well over a decade ago.
As part of this search for understanding, I came across people that believed in creationism. At first I couldn't really believe it. I'd never heard of anyone who actually thought the world was only 6000 years old, Adam and Eve were real people, or that Noah's Ark was a literal, historical event. Even at a young age, I always considered those Biblical stories as being mythological, in the same way I viewed myths of other cultures. As I also had an interest in science, I began discussing and ultimately debating these topics.
What I found particularly egregious about the whole thing was the apparent influence creationists had over the American school system. From trying to push these concepts into schools and/or trying to silence the teaching of mainstream science, it seemed that creationists were (at the time) a surprisingly powerful force in American politics.
Eventually I hit upon a brick wall when I started to try to discuss applications of evolutionary biology, particularly related to phylogenetics. This was something which creationists had no answers for. So around a decade or so, I left the discussions and debates realizing it was going nowhere.
I only recently returned (under a different account) out of curiosity and to see what has transpired since. It's a bit remarkable to me to see the exact same discussions and debates from over 10 years ago, not to mention a lot of the same members doing the discussing. Nothing really has changed.
Anyway, that's why I am here.