Companies do layoffs in a dehumanizing way largely to avoid lawsuits. If individual managers communicate it, some might say something wrong and invite a lawsuit. Even if they don't say anything wrong, some of the people being laid-off may misremember what they were told, and without a clear record of what they were told, they might win a lawsuit. In contrast, a mass email laying people off has the advantages that lawyers can review it beforehand, and it is clearly document exactly what was told to the employee in the course of laying them off.
The other part is they want to lay everyone off at the same time, so people aren't finding out at different times and wondering if they are safe or not. An email, sent to everyone at the same time, achieves that much better than trying to rapidly schedule a lot of simultaneous meetings.
Perhaps there is a better way, but that is the conventional wisdom that I've heard from folks planning layoffs.