> then why do poor people shop there?
While the typical viewpoint is that "poor people" shop there, that's actually somewhat of a misnomer.
Most dollar stores in the US are located in rural locations, and in part because a lot of rural population is also "lower income" they get the appearance of "only the poor shop there". But the part the folks who label the stores as "for the poor" often overlook is the "ruralness" aspect. That dollar store might only be a five to ten minute drive away to grab something, meanwhile the Walmart or Target or other, that likely has the better deal (the 128oz of Tide for 9.99 vs the 8oz of Tide for $1.50) is a forty-five minute drive away one way. So couple 1.5 hours round trip commute, plus fuel costs for that 1.5 hours, and you start to see why folks would more likely shop at the dollar store vs. the store that actually gives them the better deal overall.
That's partly the "magic" of the dollar stores for corporate. They sprout up like weeds in rural areas much like Starbucks sprout up on every corner in cities. And they capture sales largely because by sprouting up like weeds, they are a shorter round-trip drive to grab sometime (esp. to grab those one or two things you forgot last weekend when you /did/ make the 1.5 hour round trip drive to go to the nearest Walmart for the better deals). These store's sales largely come from the 7-11/Starbucks method in the city: convenience.
And couple the above with the fact that in rural USA, there is effectively zero public transportation and very little in the form of uber/cab companies, and so if one does not have a car, one may be stuck shopping at the dollar store 5-10 minutes away even if one knows the stores are gouging.