There could be several reasons why some recipes do not specify which type of flour to use:
Flexibility: Some recipes are intentionally designed to be flexible, allowing the cook to choose the type of flour based on personal preference or what is readily available. For example, a recipe might work well with either cake flour or all-purpose flour, so the author leaves it up to the reader to decide.
Regional variations: Different regions may have different types of flour available, and certain recipes might be more common in specific areas. To accommodate a wider audience, recipe authors may choose to omit the specific type of flour and trust that experienced cooks will use what's typical in their region.
Adaptability to dietary needs: By not specifying a particular type of flour, the recipe can be more inclusive and accommodate individuals with dietary restrictions or preferences. For example, if someone needs a gluten-free option, they can choose an appropriate gluten-free flour.
Tradition and assumption: In some cases, a recipe might be so traditional that it is assumed the cook will know which type of flour to use based on common knowledge or cultural practices.
Oversight: It's possible that in some cases, the recipe author simply forgot to mention the type of flour, or it was an unintentional omission.
Despite these reasons, it's essential to use the appropriate type of flour for a recipe to achieve the desired texture and taste. If you encounter a recipe that doesn't specify the type of flour and you're unsure, you can experiment or look for similar recipes to get a sense of what might work best. When in doubt, all-purpose flour is a versatile option that can be used in many recipes.