You absolutely can do that and plenty of people do! But you have to remember that boxes are expensive now a days!! An average box is in the 100-150 range. Plus your card stores can't compete with online prices because of their overhead costs. And what if your card store sells out of that product?! Then you have to wait to order it or buy it online, pay shipping, etc.
Now when it comes to the high-end products that cost 400-600 a box, not very many of us can shell out that kind of money to take a risk of getting "skunked" on a box. That's another advantage of doing box breaks. The site gives you access to super high end products at only a fraction of the price! Yes you have a smaller percentage chance of hitting a card but that's what you pay for. We've seen 1 spot produce some of the biggest hits! And if the box didn't have a decent hit in it, you'll be saying "man am I glad I didn't shell out $600 for that box"