I am going to interpret your statement as having the characters possess only a static set of powers and abilities, with no advancement.
There is actually no specific reason not to run campaigns with static characters. Such games can indeed be entertaining. But there are a few drawbacks.
- Games will get boring, since after a while, you will run out of opponents you can reasonably use. And long before than, your party will just get bored with their static characters that can only do things they have done already.
- It can strain credibility. Lets say you have a static party is challenged by a handful of Ogres, and you want to have a very dangerous lich face the party. Its hard to sell the lich as dangerous if its total combat ability is not that much worse than a hand full of Ogres
- If the party is static, You cannot really give them much in the way of reward for a job well done.
Also, I think I can say with some accuracy why DM's prefer low level games.
Not all campaigns run for 14 years. Games often fall apart, and most campaigns just do not run that long. When DM needs to restart the campaign, he will usually do it from 1st level. This tends to make DM's very familiar with low level adventuring.
But when a game gets to higher levels, it gets harder to run the game very well. Most DM's will know the low level beasties quite well, but they wont be as familiar with CR 10 monsters. Statting out a 1st level Rogue is easy. But statting out a Char level 10 Rogue can be a bit harder, since you have to account for the stat increases and figure out appropriate magic items for them. For Fighters, the feat list is impressive, and for Clerics and Wizards, there are that many more spells to work with. And it gets even harder if you want to multi-class the character. Its not easy to just throw together a Ftr 4 Rog 4 Duellist 2.
Another factor is that alot of DM's will get caught off guard when the party, at about levels 5 through 7, start tearing through encounters using the kinds of monsters he likes. Even if the DM thinks he has made the encounter level appropriate. There is a certain point in that level range where the party's total power level suddenly spikes. Fighters start getting iterative attacks, Rogues are scoring 3d6 extra damage on a sneak attack, and Wizards are lobbing around the Fireballs. This can make fights that the DM expected to be difficult suddenly a great deal easier.
And the other part is that the game can also seem to slow down in general. People will start having to look up rules for things they just dont know. At 1st level, not many players will bother with a Disarm or Trip. But at level 5 or 6, it makes sense, because your going to be going up against opponents that simply dont fall down in 1 hit. The DM will also be using monsters that are more complicated then Orcs, Goblins, Ogres, and the like, which is yet more stuff to look up.
The last thing I suppose is that at low levels, character death for the players may not be a big deal. Rolling a new one is pretty easy. But f you need to roll a higher level character to replace a casualty, as noted for NPC's, its more time consuming. And if a player decides to go for an entirely new type of character, he can introduce many new rules or abilities that the DM is unfamiliar with. And this will drive a DM nuts.
Running a higher level game is quite simply a great deal more work. But I am also convinced that if more DM's stuck it out and ran games of that level range, that they would become comfortable with the unfamiliar material.
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players. He got up to lvl 16 (arbitrarily, sometimes leveling us up twice in a session), and then went completely insane, and just tossed a CR 30 undead god-child at us that wanted to undo all of existence.
