They have enough employees to build native apps that run super quick but are still seduced by the web portability argument which, as we all know, is mostly untrue even now and which introduces all kinds of non-deterministic latencies/errors, which cannot all be handled neatly.

To be honest, this is the same in almost all apps that have any more than 10 developers working on them (my estimate!). Death by dependencies and a lack of coherent design.

As someone else said, though, some things like fastmail work OK in the browser so it is possible.