I work for Big Tech. It has its faults, but blaming a design monoculture on it isn't fair. Design trends are bigger than a single company, and while there are definitely enough companies to see some amount of variation in UI and style, they tend to look similar because of tech's ubiquity. You don't want to be the company big or small that goes off in a different direction and can't get traction because you're using design patterns users aren't familiar with. It's like the metronome synchronization experiment: even a fully democratized space can trend towards monoculture.