What is the reason?

I think that forum sites do not do this because if they did the entire community would be flooded with just referral links and would not give the site any kind of credibility or money. The way I see it, the more referral links, the more people will go away from the forum site. It's like if you own a store, and one of your customers comes in and tells another customer that the meat is cheaper at another store! Your losing a customer, for if nothing else one product!

There would be too many pointless threads created if that was allowed. There is alot of other places that allowed that type of post to posted. Maybe try others.