Why do sleeves need separate cutting out and then sewing; why can't sleeves be part of main body pattern piece?
Peggy, thank you for your question.
To best answer your question, I have posted an image courtesy of thefoldline.com. In the photo you can see that most of the styles are "sew in" sleeve styles, the reason for this is our bodies are not flat. In order to have a garment fit well, we need to be able to shape fabric around our curves, in this case, the shoulder and back. The only way to do this with most sleeve styles is to cut the sleeve separate from the bodice. This way you can manipulate the fabric with tucks, gathers, and curves for style and fit. The exception to the rule would be a dolman/batwing sleeve or kimono style sleeve. These sleeve styles are cut as one piece attached to the bodice and they are typically loose-fitting styles. Keep in mind this style is not for everyone, but if you are a beginner sewer, I recommend trying this style to perfect your seam techniques first and then move to a fitted style garment.
I hope this answers your question. Thanks again for connecting. Let me know if you have any other questions.