) What constitutes the incumbency advantage? There are several factors: name recognition, money, experience and the powers and privileges of the office.

The first and perhaps most important reason that incumbents are reelected so easily is that they have been serving in the capacity they seek and they are known by their constituents. Even if people don't otherwise know whether they agree with a candidate, if they recognize the name they may be more likely to vote for that candidate. Incumbents can associate their names with the office to present a formidable wall for challengers to scale.

Second, because incumbents are in office and able to operate as powerbrokers among likely donators (and because they are, after all, more likely to win), they can raise more money, to use in campaigning to make their names even better known, to further ward off challengers.

Third, incumbents have records that can be judged and experience they can point to in order to prove their competence for the job. Of course, they may have to defend their records, but skilled politicians do that for a living. The experience counts for extra votes among the electorate and the fact that incumbents can point to ways that have already benefitted their districts is a big reason for this.

Fourth, incumbents have the ability, under the franking privilege, to send mail to their constituents free of charge. This allows them to further exploit their already-significant advantage in raising money to campaign. Due to the high cost of campaigning for public office, therefore, only very wealthy or very well-connected candidates can afford to challenge an incumbent. Also, incumbents have local offices in their districts that serve as liaisons to their constituents and can be easily converted into a campaign force.

Interest Groups

People join interest...

It is not obvious that people should join interest groups. Once a policy is passed everyone gets to take advantage of the benefit, regardless of whether they contributed to the cause of passage. It might make sense, therefore, for people to simply sit and wait for the crowd to do its work and then take part in the collective benefit. This is the essence of the free rider problem. And in fact, such a situation is sometimes the case. For example, in the case of a worker in a right to work state, he might join a workplace where collective bargaining has taken place so that he gets the benefits, but he may opt out of participating in the union. However, Americans generally are a passionate and committed people who want to have some say in their government. Their interest in self-government is, perhaps, at least as strong as their apathy about working in groups to achieve ends, and so they join in ways that make them feel comfortable, by contributing money or knocking on doors or volunteering their time and money in some other way. The culture encourages and supports such acts of civic duty, and people find themselves motivated to carry their own load.
Another reason that people join interest groups is that there is a flipside to the free rider problem. This is: Not only does everyone benefit equally from a policy whether they acted on it or not, everyone also suffers from it equally. Since the nation is a nation of closely divided ideological forces, with approximately half the people thinking the government should govern one way while the other half wants another direction, people stay involved in interest groups in order to make sure that their side wins as often as possible, that when they don't win they are able to lose less severely.