I would attribute it to physics.

If the entire page is rendered on the server, the information required to do so is presumably reasonably approximate (same datacenter). If most of the page is rendered on the client, the information needs to be pulled in from arbitrary physical distances.

At some point the engineering really is this simple.

Which is also, or at least has also been a culture thing, because of the

"Oh no, we can't possibly do the computation of all that on our backend! Yikes! Let that run on every single client instead! Let users spend their own compute over and over."

culture.