That might be some of it, but I think there are even more quotidian answers: large companies often have fixed design budgets (by virtue of having a design staff

That might be some of it, but I think there are even more quotidian answers: large companies often have fixed design budgets (by virtue of having a design staff sitting around, waiting for things to do) and are generally under pressure from investors, etc. to demonstrate market fit and relevance. In other words: "what are you paying all these designers for, if you aren't going to have them match the latest trends?"