Maps is core to privacy and utility for their whole software ecosystem. They offer maps free to devs on iOS so they're not forced to leak a boatload of data to Google or pay big API fees. It's a built in that thousands of apps use.
Thousands of apps aren't using search.
There's no particular reason for every app to search the web, though. There's plenty of reason for a wide variety of apps to need maps.
Maps are a broad value tool. Searching the web isn't. A web search engine is just a search engine. It's not a meaningful value add to any arbitrary developer who wants to build apps for iPhone.