It's funny to think that the fantasy/speculative fiction trope of "precursor civilization that has advanced tech" was probably grounded in the historical memory of people happening upon ruins, weapons and artifacts of extraordinary craftsmanship.
For several centuries after the Western empire fell apart, Europeans lacked the capability to build settlements or structures anywhere close to the size typical of classical Roman civilization. Populations were depressed by pestilence, social order had declined, and most Europeans were within a short distance from a former Roman settlement of a size larger than they'd ever seen populated in their lives. The Eastern empire was a little different, but Western Europe was pretty much fubar in the early middle ages.
Although technology steadily progressed throughout the middle ages, it took centuries for population and social organization to return to anywhere near classical levels. If you lived during the early middle age, it was an unmistakable fact that a great "precursor civilization" capable of wonders far beyond what your own civilization was capable of had lived and fallen long before you were born. With colossal roman ruins scattered across Europe, there was no need to look to the bronze age for a fallen civilization.