Maybe you just don't read many books written after the year 2000? It was a pretty common word even before ChatGPT: https://books.google.com/ngrams/graph?content=delve&year_sta...

But perhaps the most famous source is Tolkien: "The Dwarves tell no tale; but even as mithril was the foundation of their wealth, so also it was their destruction: they delved too greedily and too deep, and disturbed that from which they fled, Durin's Bane."