Why Can't (B) Actors Be President?

I can't begin to list the number of television episodes/movies I've seen which include this line or a variation thereof: "In the future, I can't believe a B-actor will be elected president!"

This exchange from Back to the Future is a good example:

Dr. Emmett Brown: Then tell me, "Future Boy", who's President of the United States in 1985?
Marty McFly: Ronald Reagan.
Dr. Emmett Brown: Ronald Reagan? The actor? [chuckles in disbelief]
Dr. Emmett Brown: Then who's VICE-President? Jerry Lewis? I suppose Jane Wyman is the First Lady!
Marty McFly: [following Doc] Whoa! Wait! Doc!
Dr. Emmett Brown: And Jack Benny is Secretary of the Treasury.

This viewpoint has always struck me as rather undemocratic. Isn't this the country where hairdressers, small business owners, dairy farmers, cops, and department Santa Clauses could all become president? Why not actors and/or singers? Are career politicians really the BEST choice?

But more than undemocratic, the lines (about Ronald Reagan; I rarely hear similar comments about Sonny Bono) have always struck me as odd: Why would actors put themselves down like this?

Then one day I realized that it is scriptwriters, not actors, who come up with these lines, and scriptwriters can be rather obnoxious, especially when they hold grudges. And nobody holds a grudge like a self-appointed "high IQ" scriptwriter who makes actors look good but whom nobody knows exists.

But, still, why would the ACTORS repeat the lines? Has no actor ever said, "Excuse me, I'm a B-actor, and I think I would have the right to go into politics if I wanted?"

It is possible that most actors just want to be paid and even possible that most honestly don't think they should be in politics. It is also possible that some actors are so sheep-like about politics, they are perfectly willing to sabotage basic democratic principles for the sake of a snide joke, in which case they shouldn't be involved in politics--AT ALL. Stop trying to save America from itself, Hollywood!

On the other hand, statistically-speaking, I'm sure there are some actors with strong political visions who are attracted to the local or National political scene. I might not vote for all of them, but I would certainly defend their right to try to get my vote.