Remember that many of the online retailers are actually local tackle shops. They do have overheads, but they have invested in their business to move with the times and ensure they keep profitable via additional online sales.
Very much a double-edged sword.
To compete with "pure" on-line retailing you put your shelf products up online at a greatly reduced profit-margin. You then have to sell many more of these to create the same profit, which consumes more overheads.
You borrow money from the bank to have a higher level of stock to satisfy the increase in online business and more labour to fulfill the ordering and mailing process. The reduction in "cash" sales and increase in credit card takes more profit away in the form of card commissions and bank processing charges.
Meanwhile your existing walk-in trade switches to buying from you online since they realise they get the same products cheaper that way, so you lose some of the "impulse" spending they would normally make in wandering around your store.
If you're lucky enough to be a big enough (and reliable) business that suppliers will give a months credit to, then that could well pay off if you manage to turn it over in that time-period, but many are not in that position.
Compared to many businesses tackle shops have a vast outlay in shelf stock (if they're any good) and have an awful lot of capital tied up for the turnover that stock generates.
I buy most of my consumable stuff from my local shop(s), but sometimes have to go online for the more esoteric items. More and more I find I buy expensive kit online simply for the savings, not that I need to buy much of it. By sticking as much as poss to quality brands you reduce the "refund" factor mentioned earlier.