Raw meat and palm hearts helped early humans survive life in the rain forest:
Brian
Plus, it’s not a guarantee that knowledge/technology will continue to exist. Invaders or conquerors can destroy and oppress a culture, a new religion can radically change a culture’s outlook or values, natural disasters can wipe out people and repositories. After just a few generations, no one would be around from when (insert technology or advancement) was around.
As long as we aren’t talking about ancient civilizations having spacecraft or computers, anyway. 
Well right. If the previous tribe was experts at making clovis points or pottery, unless you keep the craftsmen who did this (as was traditional in later empires, carrying them off to your capital to serve your leader), then that technology is lost.
This sort of thing likely happened thousands and thousands of times throughout history.