Because you don’t want to look like a phishing website.
Publishing complete contact information that is easy to find and use is critical for the credibility of any business or a organization with an online presence. It is easy to do, and yet one of the most common glaring deficiencies of websites.
By far the most common kind of fraud on the internet, by some huge margin, is that websites that pretend to be legitimate and ask you for stuff: the person asking for your credit card number isn’t who they say they are. And one of the main ways that these scumbags conceal their identity is by simply failing to provide a real address. (They tend not to provide a false one because it’s too easy to check — if there’s nothing there, there’s nothing to check.)
Address-silence has become pretty much synonymous with dodgy websites. You don’t want to seem like one of them.
What if you’re not an ecommerce website? What if you’re not even taking credit card numbers? Doesn’t matter. It’s an association thing. If you’re asking anything from your visitors, you need to not seem anything like these dodgy websites.
Like all design issues, it’s not what you are, it’s how you seem. If you use a presentation that is like the presentation used by many, many dodgy companies, then, alas, you will seem dodgy too. It’s one of the oldest tricks in the design play book: if you want to seem classy, then you don’t use the same fonts and layout tricks used by used car dealers.
And if you want to seem like a “real” organization on the internet, then don’t do what fifty thousand websites made by Slovakian crime syndicates have done, and just publish an address.
It’s also just friendlier
Failing to publish an address is also synonymous with big companies who deliberately try to make it difficult for you to contact them — one of the modern consumer’s biggest pet peeves. Hiding behind contact forms and making it hard to find a simple, useful contact page is highly characteristic of institutional red tape and not really wanting to deal with the public.
Consumers overwhelmingly prefer to see an actual snail mail address, an email address, and a toll-free phone number ... for exactly the same reason that they like “getting a real person right away” when they call.
Humans are social critters. Scientists believe that our brain size may have evolved primarily for the purpose of processing social information. Online, this translates to a keen interest in the “contact” and “about” pages of websites: people almost invariably go to those pages to find out who they are dealing with … or thinking about dealing with. If you want to build a relationship with them, don’t disappointment them!