Because part of being a CEO is having social skills, and doing what you suggested is extremely socially inept.

Explaining why layoffs needed to happen, expressing remorse about it, and giving a good severance package is what serious adults with empathy for their employees do.

It's also a good way to maintain at least a neutral relationship with everyone you're laying off. Compare that to a company that coldly fires you with minimal severance, and delivers that message via some low-level lackey (not the CEO) – you, as the former employee, want nothing to do with that company ever again.