I'm not actually a web dev and I think they would gain more from having a big standard library but I'm fairly sure the actual decision makers at the companies that own these websites don't agree with either of us.

They basically want to squeeze every bit of performance from the things that they think don't bring business value so that they can afterwards burden these websites with every advertising, tracking, compliance, cool animation (from their POV), etc library and tool on the planet. Basically bloatware.