People are strange. My family all have iPhones, we all use iMessage, yet for some reason we have a group chat on WhatsApp which is where my sister and mam share

People are strange. My family all have iPhones, we all use iMessage, yet for some reason we have a group chat on WhatsApp which is where my sister and mam share photos of my nieces. The functionality is no different to iMessage. I don’t know why this happens. It just does.