Why not "Win by Two"?

[Beer and Pizza]

I think the World Series (and other playoff series in Baseball and other sports) would be better if there were a requirement that the winner win by a margin of at least two games.

In other words, the series would end when the game count is:
4-0
4-1
4-2
5-3
6-4
7-5
etc.

I know a series could drag on a bit occasionally, but is there any other reason not to do it this way?