When I first started learning game engines there were a lot of "understend a game engine by building one" courses in various forms, and a lot of open source projects (plus the closed source/never published ones I don't know about) that where the implementation of an engine or a kernel or a database based on a known book on the field. So it's at least not entirely a result of agentic coding and AI in general, even tho it probably incentivized the phenomenon