Because there are like three companies in the entire world that make RAM chips, they do so for almost every electronic device in the world and they concentrate most of the know-how and the patents, so there aren't many chances of a fourth competitor surfacing any time soon. It's easy for them to reach an agreement on minimum prices.
The motherboard market, on the other hand, has at least six or seven widely known companies, with others that are not really well known, but make many MBs for OEM systems. There is a lot more competition.
Some of us tolerate this because there isn't much we can do, besides not buying, and sometimes we need to build a new PC. Others just have more than enough money to spend on whatever they want, even prohibitively expensive computer parts, and/or don't really care how much it takes to get the latest and greatest (that doesn't mean they won't try to find a lower price first, however).