That would be the case if each rider would have the same chance of winning and each rider would be there to win. However, for many riders, the national championship are not their main focus. They might be exhausted from a long program of races in the previous weeks and are not at their best. Or they might be preparing for other more important races in the coming weeks and are not yet on their best, for example when coming from a training camp at height, or are unwilling to take extra risks to prevent injury. And, of course, some races are better suited to certain riders than others.
All these factors mean that there are a lot of riders that know they aren't there to win, but would be willing to help another rider. For example, one who has helped him before, or a team mate, or one would could beat a rival, or one who is an underdog, and so on.