Its because the people up top, like to cut costs. Look at Walmart site. It is horrible, I hate it. They are the number one retailer in the world, yet spend as much on their website as you will on a lawn mower there.
- Building it is not easy, most employees are not being paid for this or it's not their job.
- Having it costs money, a cost which is difficult to justify as the Internet remains a sketchy, unproven source of revenue at best.
- It is far from essential, business is (and has been) done without.
Not to say a business should not have a site, but it is an add-on and to be sure, one should neither depend on it nor should one put more than spare time and money into it (because it may provide little or nothing in return).
These are some of the main reasons a lot of sites on the Internet are still being built / maintained mostly for fun, or as a hobby.
I think it depends on the business that you are in. Walmart has storefronts. You know where they are and the types of products that they sell. I don't think a company like that can benefit from a website. Now a company like Old Navy or Speigel or any of the other clothing type businesses flourish because of their web related sales. I think it accounts for almost 60-70% in most cases. Walmart type stores also benefit from impulse sales thats why they don't want you on a website....they want you in their store.