Because they like them. I was more involved in consuming anime when I was younger and it was just based on characters you liked for whatever reason. Some people go to more extremes than others but that's the same for any hobby.
I picked mine for nostalgia, but I like Earnest P. Worrell because he's always making some [overly complicated invention]https://youtu.be/uYMw_dlnVH0?feature=shared) that didn't need to exist, and he switches between multiple characters depending on the situation. I also like his grandiosity, integrity, and drive to do what he thinks is right. He remains positive in difficult situations, and I like his energy.