I think a lot of what happened is that Google and the like never sufficiently open sourced their monorepo secret sauce, and that led to (dumb) engineers adopting familiar structures, like GitHub orgs with many different repos that they could easily deploy in a 1:1 fashion with cloud tooling. Fast forward a decade, and the "microservices" monstrosity has become normalized.

i think also the huge VC driven hyperscaling moment in software development and creating all these huge companies who were hiring 100s of engineers per month who ran fast and broke things meant that it was easier to make new hires productive is every new team essentially had a green field project vs onboarding to monorepo and getting familiar with everything