This is so because political scientists simply study politics and the political process while politicians have to participate in that process. These are very different activities.
For example, Nate Silver and his 538 blog colleagues study one particular aspect of politics. They study polling to see how well they can predict what will happen in elections and to see what the polling can tell them about politics in the US. They are very good at what they do. As seen in the link below, one thing that they have found is that President Obama’s early ads did not really seem to have a big impact on public opinion in the way that many commentators thought they did. This is a very important insight about how elections work. Many politicians would not have the intellectual ability or the interest that is needed to carry out such work. This is one reason why not all politicians are political scientists.
On the other hand, Silver and his colleagues probably could not do what politicians do. Politicians have to have a very different set of skills than political scientists do. Politicians have to have the charisma and the personal skills needed to get elected. They need to have the political and interpersonal skills needed to get legislation passed. These are not things that necessarily go along with the kind of brain power that political scientists need to have.
Political science is an academic pursuit, not a hands-on process of getting elected and making policy. Therefore, the skills that a politician needs not the same as those needed by a political scientist. This is why politicians are generally not political scientists and vice versa.