So do you also sometimes run into 15th-level monsters when you're 1st level? Is the BBEG sometimes just too powerful for you to bring down?
Oh, abso-damn-lutely!
Not only do I have DMs that throw that kind of stuff at me, I throw that kind of stuff at my players when
I GM. Part of life- thus part of being a PC in my campaign worlds- is recognizing authority and power, and when someone with one or the other or both is simply too much for you to handle. That means its time to talk, walk, run, or surrender instead of fight.
When the BBEG has you outclassed, you bide your time, go on journeys of self-improvement (OK...graverobbing, looting and pillaging, but in a GOOD CAUSE!), and return to confront the BBEG when you're more powerful.
It worked for George Lucas' storyline for Luke Skywalker, after all. Why shouldn't it work for you and yours?
And the same goes for non-BBEGs as well. Those city watchmen or the caravan guards aren't necessarily pushovers & idiots in my campaigns. They're going to use equipment and tactics appropriate to their training and level, so don't think your party is just blithely going to rob even the most evil merchant's treasure wagon.
Not like this should be a surprise to veteran D&D fans. If you look at 1Ed personalities and modules, there were definitely encounters & critters best avoided instead of confronted.
Stealing pages from Indian, Greek, Norse and Japanese legend, Bahamut's avatar routinely wandered the world disguised as an old man attended by a flock of seven little golden birds...each a polymorphed Gold dragon. Wander through Expedition to the Barrier Peaks and you'll encounter the police robots soon enough- you don't tangle with
those without being well prepared.