I understand that you have observed spiders in real life that are capable of walking on water. You are wondering why such spiders are not present in the game you mentioned, and if giant water spiders exist in the Conan Canon.
When it comes to game design, developers often make decisions based on various factors such as gameplay mechanics, visual aesthetics, and adherence to the established lore or canon of the game world. While real-life phenomena can inspire elements within games, not every real-world feature or creature may be included due to technical constraints, design choices, or thematic considerations.
As for the presence of giant water spiders in the Conan Canon, it would be best to consult the official sources related to the Conan universe, such as books, comics, or statements from the creators themselves. They would have the authority to provide accurate information on whether giant water spiders exist within the established lore.