Why don't websites have a "description" meta tag in the head section?

There seems to be some evidence that the meta description is useful at least for some search engines.

In general, meta tags are not useful. Google is over 90% of our traffic, and they only use the meta for site summary in the case where you aren't listed in DMOZ.

http://googlewebmastercentral.blogspot.com/2009/09/google-does-not-use-keywords-meta-tag.html

For example, we do sometimes use the "description" meta tag as the text for our search results snippets, as this screenshot shows:

_{(source: blogspot.com)}

Even though we sometimes use the description meta tag for the snippets we show, we still don't use the description meta tag in our ranking.

We are listed in DMOZ so I see no reason to have 100+ bytes of meta site description in every page we serve over the wire. The benefit is trivial, and the aggregate impact is large.

As an aside, when you google "stackoverflow", where does the "A language-independent collaboratively .." description get pulled from?

See here:
Does an entry in DMOZ significantly help the ranking of a web site?

As mentioned, the description you mentioned is pulled from DMOZ. However, one reason they may not have a meta description is because StackOverflow is primarily user-generated content. As such, creating a relevant meta description isn't the easiest task. They may have thought that Google would do a better job of programmatically creating a search engine results snippet for any given question than they would have.

Google will use your meta tag description for the listing, even if you are listed in DMOZ.

It will probably NOT help you rank higher, but it makes a lot of difference when it comes to conversion. That meta tag description is what people read when they decide to click on your website in the SERP.

When I search for something on Google, about 4 results appear on my screen in Firefox, and 6 in Chrome (I have to scroll down to see the rest). Which of these results I click on depends mostly on the title and the description. IF these results don't seem to be what I am looking for, I will scroll down for more.

Treat that metatag description as a free text ad for the page.