Because McAfee pays them to use their antivirus on Samsung devices. It's a great way to advertise their antivirus. Same was the case with Clean Master. If you don't know, Samsung once used Clean Master on their storage cleaner app. They removed it after a lot of backlash. They will eventually remove McAfee too as a lot of people hate it especially the Americans (you can read on Google why). All these OEMs dont add antivirus not to protect the device (antivirus is pretty useless on Android), but to earn as much as possible. https://www.reddit.com/r/Android/comments/68rtn1/clean_master_is_what_samsung_uses_for_their/
