At the end of this article I will present empirical evidence showing the lack of a bonus from outgoing links, but first I want to explain why they're not used by search engines.
Despite what some webmasters believe, search engines are not in the business of refereeing webmasters. They are in the business of providing as useful a service as possible to searches, as such if something doesn't improve relevancy it probably isn't going to be done.
One thing we learned from the late 90's is that you cannot trust webmasters when it comes to ranking their own sites. Meta tags were spammed and stuffed like crazy because webmasters had control over them and this forced search engines to come up with something better, and they did, off-page factors like incoming links (or Google's PageRank). By looking at things not on the site itself, thus things out of the control of the webmaster, search engines regained relevancy. Its true, a savvy webmaster can control a portion of his off-page factors by managing his internal linking structure or by having more than one site and manipulating the results that way. However no individual webmaster has control over more than a fraction of a percent of the Internet, if that. Whereas a webmaster has absolute 100% control over their on-page factors like meta tags, and outgoing links. So while the potential for abuse of off-page factors is still there, its almost infinitely lower than the potential for abuse of on-page factors.
The point of this being that outgoing links, because they are on-page factors, are easily abused and thus unlikely to add to relevancy. Also they are so easy to add and cost webmasters nothing to add so if they did indeed help everyone would do it and thus again you'd have no increase in relevancy, just an increase in a baseline score.
Then there are usability issues. Some people claim that hub sites are more usable or can be more usable than authority sites and so why wouldn't search engines rank them well? What these people fail to grasp is that SERPs (search engine result pages) are hubs. That's right, a search engine makes it's own hub. If a search engine knows the top 100 authority sites on a topic, why would it instead link to a hub site that may or may not include the same 100 sites? At best its going to require extra clicks on the part of the user to find what they want, at worst it's not going to include all the best authority sites.
One search engine, Teoma, does list hub sites, it lists them in a separate result set on the side of the page. This is really the only logical way to do such a thing without a loss of relevancy.
Incoming links are counted as votes for a site, but what are they voting on? Usefulness. A site that is useful is going to get incoming links, a site that is more useful is going to get more incoming links. The number of incoming links you have, as compared to other sites in your field, is generally a function of site age multiplied by usefulness.
Ya, so what? Well this fact makes the whole rest of this discussion somewhat pointless. The #1 claim by any proponent of the outgoing link myth is that outgoing links can make a site more useful and so Google and other engines should count them. They don't realize that by counting incoming links search engines are already getting a measure of usefulness as judged by real humans and it'd be foolhardy to question that measure with some arbitrary algorithm that has to guess whether or not a certain component, like outgoing links, adds to the usefulness of a site or not. Certainly not all outgoing links are useful, so if a search engine were to give bonuses for them how would it decide which pages are deserving? Nothing will be more accurate that real human opinion, and real human opinion is what they get by checking incoming links.
There are two components to a search engine algorithm. One component determines what a site is about. The other component determines how useful that site is on that topic. Counting incoming links as a measure of usefulness makes pretty much every other measure of usefulness irrelevant because a sites that are more useful as judged by real people get more incoming links. Measuring incoming links is the only technically feasible way of adding human moderation to search results, and it works.
So if outgoing links do indeed add to the usefulness of a page, it's already being counted by measuring incoming links. Second guessing that measurement will only lead to a decrease of relevancy.