Why YC companies never hire remote worldwide?

It's non-trivial and often expensive to hire international employees (legally [0]). $Billion tech companies can afford to do this (staff HR teams and shell out visa-related fees), whereas lean, small startups cannot.

[0] There's a significant difference between "employee" and "contractor".

Loss of productivity. It's a lot easier to talk to people face to face and whiteboard a hard problem together than it is to do it via video conferencing.