I can think of a thousand reasons, but for a few:
- 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.