They don't have to do it at a reasonable price just as long as they can do it NOW and, for expensive parts, I can return it if it's defective or the wrong one.
That's the secret to Best Buy's success. You can always beat the price, usually by a lot, if you're willing to wait a couple days and have a return hassle if there's a problem.
As another poster noted, if I have to wait for any part of my project then I'll order it all online and wait.
Radioshack is too small for its hobbyist demographic. They can't carry enough stuff. With arduino and its ilk, and the rise of the new maker demographic, the required selection of parts has greatly expanded.
The mall locations are killing them too: the hobbyist shopper is a destination shopper not a walk-in. Having the store in a mall adds cost and limits space with no up side.
They won't be able to sustain cell phone sales in a mall either, not in direct competition with the apple store, the verizon store, etc.