iserith
Magic Wordsmith
I think this matter goes a lot deeper than just not reading optional rules or the like. Many experienced DMs just assume "they got it," when it comes to new editions of D&D and they run their games pretty much as they've always done. So the thinking is, "who needs to read the DMG?" Now, to be fair, some DMing skills are universal and portable from one edition to the next. But certain things aren't as compatible with the new edition and the games might not run as smoothly as one would hope if treating them the same as some other game. Reading both the PHB and DMG helps paint a more clear picture in my view of how the game is designed and what approaches to playing and DMing are the best fit to make the system work optimally in play.