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."

As a non-native speaker, I read a lot of fantasy and science fiction books in English. I use "delve" regularly (I wouldn't say "frequently" though). Not sure if it's Terry Pratchett's Discworld influence, but plenty of archaic sounding words there.

I did not even know it was considered uncommon and archaic, tbh.

People from different countries especially where English is not their first language often have more esoteric words in their vocabulary.

I guess there may be regional differences but delve is a commonly used word for native speakers.