Demons are generally described as not having free access to the Prime Material.
Liches probably have better things to do than waste their power destroying a meaningless town.
Anything with a reasonable amount of intelligence is going to realize that running around mindlessly from town to town destroying everything in sight is going to get it killed eventually. Powerful heroes probably don't spend much time in Littletown or Otherlittletown or Thatlittletownovertherethatslikelittletown but they're likely to take notice if these towns are all destroyed. Anything that goes on a berserk rampage (short of perhaps the Tarrasque) is going to end up being hunted down and killed sooner or later. That's assuming it isn't unlucky enough to stumble across the one town out there with retired but still powerful heroes in it. Even liches don't know everything, and what you don't know can definitely end you.
That lich didn't spend all that effort becoming immortal just so he could throw it all away on some cheap thrill. Even most demons are intelligent enough to realize that a trip to the Prime Material is too valuable an opportunity to waste on meaningless destruction. Why bother with Littletown when by finding and destroying the Great Seal it can finally undo the barriers that prevent his 10k cousins from coming here as they please. Of course, if Littletown happens to be on the way to the Great Seal, or someone there is rumored to know it's location, then that's a different matter entirely...
Effectively, creatures that are intelligent will usually feel that Littletown isn't worth the little effort it would take to destroy (no gain, but it might draw the attention of dangerous and powerful forces).
Less intelligent powerful creatures probably wander about hunting in their territories, deep within the lost regions of the world. They don't know about Littletown and could probably only find it by stumbling across the town by chance (a small chance, since it's a little town). Stupid but powerful creatures might love nothing more than to devour the entire town, but the townsfolk didn't survived this long by building their homes next to a Remorhaz nest. They built them somewhere relatively safe, far from the most dangerous monsters. (Littletown survives by remaining unnoticed).
Finally, on rare occasions it may well be that monsters have attacked Littletown. Many townsfolk were likely killed, but likely at least some escaped. The survivors would have had to choose between either becoming refuges (abandoning their homes and worldly possessions, and casting their fate to the winds), or hunkering down and rebuilding what remained. Those survivors who stay to rebuild prevent the destruction of Littletown from being complete.
