I wonder if there are any duckduckgo/other search engine devs here to answer; why not keep a limited own index? Like, wikipedia, stack overflow, build up a cach

I wonder if there are any duckduckgo/other search engine devs here to answer; why not keep a limited own index? Like, wikipedia, stack overflow, build up a cache of own-scraped sites for the top hits to popular search queries? Am a being naive with how much is possible without going all-in and building an index of the whole web, or is it possible to have some kind of in-house fallback?