History has shown it to be true
Yes one time I threw 27 heads in a row .... that is a historical record .... do you believe that would imply the next throw will be a head???? If you do are you willing to bet the ranch on it????
Your explanation why incumbents don't want challengers is like a pundit in search of a justification of an opinion. While a campaign is not a simplistic coin toss, it is a dynamic system with a set of complex variables, it would imply reduction to previous outcomes doesn't make sense. Every election cycle is different.