re: Why do teams punt in the preseason?

Actually, yes punting is illogical on most 4th down situations, at least in the NFL. Read Do Firms Maximize? by a Berkeley statistics professor. He analyzes data from last decade of NFL games to point out when its better to kick than go for it.

Unfortunately, coaches would rather use rules of thumb developed decades before the development of modern offense.

4th and less than 2, you should never be punting. and you should never kick a field goal from inside the 5. At least not until the end of the game when situations make 3 points or variable value.