Why do apps like WhatsApp, Facebook run background services in android even though their apps on iOS without backgroung services What is the reason for apps like WhatsApp running background services in andrioid even though in iOS they work smoothly without any background service
Because they can.

In Android you can create background services and do a lot with them, whereas in iOS Apple does not allow you to do this. Both use push notifications to get your new messages when the app is closed, so the main functionality of the app works fine regardless, however the apps may be doing more processing or monitoring, or whatever Facebook wants it to do in the background while you're not using it.