time के लिए in morning या in evening का ही ज़्यादातर इस्तेमाल क्यों होता है am या pm का नहीं

am aur pm ka use particular fix time ke liye hota hai. . but jab kisi event ka time sure nahi hota tab in morning,in evening use karte hai. .

because am and pm mean that in it also include morning and noon,evening and night.