Interesting. Are big tech companies less generous than big engineering firms in other fields? When companies like BP, Boeing, etc. have had layoff rounds, 3 months is just about the minimum package for an involuntary termination, and it's common to first offer much higher voluntary packages to reduce headcount without having to actually fire (sorry, "terminate") people, especially longtime employees. My dad accepted a voluntary severance package equal to 18 months' salary some years ago (something like "1 month per year of service up to a cap" is a common formula for voluntary offers).
I think this is in part due to what you mention: the morale of the existing employees can be significantly impacted by how their former colleagues were treated. Many people know each other out of work, so escorting the employees out of the building by security with no warning is not going to successfully keep them from talking, and if anything will probably just make them more resentful when they meet up for a beer after work to discuss how the layoff was handled. The former employees may also turn up in other situations (college recruiting, industry organizations, consulting/sales, etc.) where it's not to your company's advantage for there to be a lot of people floating around who hold a grudge about the manner in which they were abruptly laid off.