Because retailers are middle-men by definition. Large online resellers just have much less overhead, so the cut they take for being a middleman is much smaller. They often also have the bargaining power to reduce their cost price with the supplier. You should look for things that are produced in large quantities locally, and find ways to purchase direct from supplier, if you want to save money buying locally rather than spending more to support local business. Buying from independent local retailers is for indirect social and economic benefit. We should all endeavour to do it as much as we can but it’s also very hard to justify when cost differences are large.