The 'why' is presumably explained by his having OCD.
Works out at 1-and-a-quarter a day every day for 50 years. Unless he's equally obsessive about being physically very active, I'm surprised he hasn't developed heart disease by now.
Why? Its not like Big Macs are that bad nutritionally. If he's skipping fries/soft drinks he's cutting out most of the actual negative of typical fast food meals as far as empty nutrition goes. I'd wager he eats healthier than a lot of people that don't eat any fast food at all.
There's a part of the video that delves into health and he's probably healthier than you. I know he's healthier than I am.
Since it doesn't have bacon or other meats that would have nitrates for preservatives he's likely missed out on the cancer from that. Same with the heavy metal accumulation if he was eating a fish sandwich instead. And its likely healthier than a fried chicken sandwich.
I'm more surprised he hasn't burned out on them, or hasn't had enough bad ones that he either got sick from or tasted bad enough that he was like "nah, think I'm good for a bit". Oh, I saw him pull one out of a microwave. So he has no taste buds.
"He used to get 2 per day since 1972, but since he retired he spends more on gas than the food, so he has now switched to getting 6 or 8 each time he comes in on Monday and Thursday. I wouldn't be able to handle a fresh Big Mac everyday for a month (actually I don't think I could do a fresh good burger every day for a month even), can't even imagine 3 day old one reheated in a microwave.