JAY,
Anecdotally, a friend of mine wrote a letter to Harav Hagaon R’ Chaim Kanievsky asking on what basis does he hold that shaving is prohibited, and he wrote back “see such and such place where my father says it is forbidden.”
If you were familiar with the sugya you would not need to ask why those who forbid it forbid it, and if you are not familiar with the sugya then of what use is it to show you sources?