The Pokémon world has its own lore and biology. Pikachu and Eevee belong to different evolutionary lines and have their own characteristics. Their relationship in the official content is mainly about co - existing in the Pokémon world, not in this way. So, fanfiction writers are more likely to stick to the established concepts.
Most fanfiction writers are fans of Pokémon themselves. They want to create content that enhances the Pokémon world in a positive way, such as exploring new regions, creating new Pokémon battles, or developing deeper friendships between Pokémon. A plot like this would be seen as going against the essence of what makes Pokémon so beloved, so it's not likely to be written.
It's against the family - friendly nature of Pokémon. Pokémon is aimed at a wide range of audiences, including children, and such a plot would be inappropriate.