I wonder if it's to do with fixed-width serif fonts looking "old" due to their association with old text printers? Now sans-serif looks 'new' by comparison. Couple that with the 'clean sleek minimalist' look that all the modern tech companies have popularized and everything's re-converged on another round of samey looks. We're ripe for another new wave any year now. What'll it be this time? Comic sans? sARcAsTiC?
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?"
