WHY, because we live in a free market. Gas station costs (what is costs to keep the doors open) vary from location to location. Property taxes, property values, insurance, labor rates, rent, utilities, etc., all vary from city to city, state to state. Gasoline delivery costs more the further the station is from the tank farm. Even within the same city, insurance, labor costs and property values vary between areas. If you have ever shopped in a really poor, economically depressed section of town and notice the prices are higher, it isn't that the business owner is ripping off the poor people, it is because the cost of doing business in a poor section of town can be more than an upscale section of town. The extra costs for insurance, security, and loss from theft and shoplifting are passed along to the customer in the form of higher prices. Finally, DO NOT wish for price fixing on gasoline (in the form of price caps) as price fixing ALWAYS leads to shortages. That economic truth works for any product.
You can use Gas Buddy from any browser on your computer or by Smartphone browser or special app. You just put in a zipcode or location and you get all of the most recent gas prices updated by users of the gas buddy site. The one thing to be aware of is that prices on the site may not show if it is low because it is a cash only price - some do, but not all and you can pay ten or more cents a gallon by credit card and not know that until you get to the station.