I tried a Big Mac recently and it's really not that great, pretty dry. The sauce is the amazing part. I prefer getting a Impossible burger from a slightly more expensive burger place instead now. Maybe I just need to buy a gallon of that sauce and pour it on there.
Its definitely the sauce and pickles etc, but for me the draw for McDonalds was the theatre and nostalgia from my childhood, mixed with the convenience. And of course, add in the fact that I used to have an extra hamburger and 10 McNuggets as standard made it all the more enticing!