That is because on the net all of your stock is in a large warehouse with a couple of people doing nothing all day but taking orders off of a PC and shipping them out.
On the High Street you've got to employ staff, you have to have a building which has to look good.
Finally if you want the passing trade a shop has to get then it needs to be in a prime location and thus attracts high rent.
If you need something in a major hurry the High Street maybe your only option.
If not, over the net will always be cheaper.