However if you're referring to the one you made to James and khandahar who are actually arguing a particular position (unlike me) I could give it a wager.
Basic political science knowledge would inform you that historically a noticeable and predictable trend of incumbents having a significant advantage is present in our system. This occurs due to a number of reasons. First, an encumbant already has a preset foundation of which to begin to mount a campaign; in logistics, contacts, and message. They've ran a campaign before, already got donors before, already have established themselves in the states politics. Money is a huge factor and incumbents routinely bring in more than challengers.
Next, simple name recognition. Being in a position for 2 / 6 years and having already campaigned once instantly raises the individual's name recognition in the state because they are a know figure. Name recognition is big in elections for your "average" voter who is not routinely up to date with politics. Its why you see the plethora of signs every election season. The more people know your name and it sticks with them the better chance they may be inclined to vote your direction as the most memorable candidate.
Those are just two right off that are very basic level political science reasoning behind the very real and studied phenomena of incumbents typically having a significant advantage in elections,
Does that mean we should have term limits? Well, thats what's being discussed here. However, that is part of the answer to the question "why don't they just vote them out"