> It is difficult to beat raster tiles in that respect.

Intuitively, why can't the local vector rasterizer do whatever the server-side tile rasterizer does, especially if both are fully custom?

Partially because they're completely different stacks -- the client side is WebGL + Javascript, and the server side is whatever they've been doing for 15 years.

They're probably missing a raqm/freebidi or something in the stack on the client side.

It can, it's just that they typically don't, because they use an off the shelf data schema for the tiles and a library to handle the rendering.

Having the data in tiles also complicates some things (where the renderer needs to consider merged tiles to get a similar result).