A strikingly large amount of remote-work offers require the person to be in a particular time zone. Managers want you to be able to communicate with the team at times that are normal working hours for everyone. Even if there are Eastern Europeans or whatever who are comfortable working nights, they still wouldn’t get hired in spite of the opportunity to pay them less.

Communication.

It's already hard to communicate with others with slack, video,etc. Now throw in various languages and the lag of third world internet and it is pure hell.

Taxation and other regulatory, bureaucratic nuances