Not saying it can't be done.
It is just WAY TOO MUCH WORK on the part of the DM. Constantly having to plan around the casters take time away from figuring out what Lord Black is doing and how many kobolds did Flamor add to his army.
Next thing you know every patch of grass is claimed by a dragon. Two steps in any direction is a horde of demons, undead, or savage humaniods. Epic armors and swords that have all sorts of houserule magic on them handed out like candy to the mundane. Dimensional Anchor mage police. The whole royal family is charm proof. Traps in the restroom. The farmer's dog is a lycantrope. Every other wizard is a paranoid tower hermit. 20 combats a day. etc...
DMing a game with casters shouldn't be a chore. Nor should I hope my players nerf themselves just to be balanced with each other. I want to DM high level but I don;t thing I can without treating one person like the "DM's girlfriend" and the other like the "DM's annoying brother".
First of all to answer one of the people who commented in wondering how many of us who don't have an issue have actually run high levels campaigns, Well I don't have an issue, several of my DMs don't have an issue and we have all run epic level campaigns.
High level games have always been labor intensive in all the editions I have ever played in or run. And it is not just because of the casters. Fighters and all their extra attacks. Monks who and rogues who can't be grappled the sheer amount of high level magic items in play at higher levels all can give DMs a headache.
Part of a DMs job is to tailor the game to his players and part of that is taking into account the different abilities each class has.
This is true at low levels as well as high levels.
Part of your job as DM is to play the bbeg any smart bbeg with resources is going to find out all he can about the people opposing him. And it only makes sense that he puts some plans into thwart them and some of their abilities.
Nice hyperbole my the way. I am creative enough not to have to resort to the farmer's dog being a werewolf to challenge my high level parties.
As for mundane characters having magic items well at high level games it has always been assumed that the mundane PCs have high level magic items. If you don't want to play that then you do have an issue because that is how DnD has been designed. And low magic games don't tend to work if you don't do something to modify all the casters.
It seems almost impossible to have a discussion on this topic with people just exaggerating like you just did.
Their seems to be an assumption that since you have an issue then it is because of the way the system is written but people who don't have an issue it is because we don't play to our full strengths or we don't DM high level games.
I want a system that allows me as DM to tailor my game I can balance things, I can say no to certain game breaking combos. Other DMs may want something that is easier in their eyes to run they want all the balance be hard written in the system. Which is fine.
I just wish people would stop with all the exaggerated hyperbole.