Why not use the sprite art that's on the main menu it looks decent
Because that art isn't sprite art. That is art specifically made to be the main menu (And I didn't make that art, it was commissioned specifically for use as the main menu screen). If I were to use that art as sprites, I would need to also get a whole ton of expression pieces several outfits, then rinse and repeat for upwards of 24 other characters. Plus, the poses would be terrible for spritework.