I have to disagree, don't hit me either haha! To some people, that is a big difference. If I can save 4 bucks on one thing and 5 on another as you said, that's money for something else, such as a meal at Wendy's that I wouldn't have had otherwise.
Also, your examples are the extremes. Yes, some drugstore products are priced similar to MAC, such as Loreal HIP, but many more are not. Brands like Maybelline, Covergirl, Rimmel, Jane, nyx, etc are all considerably cheaper. For instance, a single eyeshadow at MAC is $14, while they would be around $4-7 for these brands. Also, a MAC lipglass is $14, and I've never spent more than $7 for a drugstore gloss. So some of these products are half the price, which REALLY adds up!
I do agree about accessibility too, the closest MAC to me is about an hour away, and I much prefer being able to run to the walmart 3 minutes from my house to get what I want. I agree with whoever said they didn't want to be bothered while shopping too. I can't stand pushy salespeople, I just want to look at the makeup myself!