Regular people probably can't use the expenses*25 approach because they have no idea what their expenses are.
I think it might be partly this, but also another simpler reason: Most people spend nearly all of what they earn because their consumption is driven by their short-sighted 'needs'.
I feel hungry. Therefore, I will go out and buy myself a sandwich and soda for $8. That motorcycle looks cool. I want it. Therefore, I will buy it.
They don't look at the alternative to buying things and instead saving the money. Their minds are simply unable to understand the "either/or" decision process. As a result, just like how an animal standing in front of a bowl full of food will impulsively eat the entire bowl full of food even if it just ate 3 hours ago and shouldn't be that hungry, the human will see their check deposited into their checking account and spend all of the money in their checking account because they don't understand the concept of how saving larger amounts of money can snowball into a powerful retirement tool, and they simply can't stop themselves from gorging on what's in front of them.
Because of this, they end up spending just about all of their income. And because of this, it's a simple process of saying 'you'll need x% (usually 80%) of your income to retire", because a majority of people simply spend all of what they earn. Therefore, at that rate of consumption, they WILL need x% of their income when they retire.