xxx6969xxx
First it's a custom, like tipping, where it looks morally bad if you only give the minimum (2 months). Future employees will care if you have a reputation for treating current employees like shit; look how many people don't want to work for Amazon.
Second is it's a Newcomb paradox. Outgoing employees have the capability to do reputational damage to the company. The press has for the past 6 years been extremely happy to shit on tech. And even though a non-disparagement agreement is usually part of the severance package (NDA, NSA, NCA already in place), the company can't possibly retaliate against every disparager.
Also, in software people may have knowledge of (or might have planted) backdoors, vulnerabilities, and weaknesses, and now, might have been motivated to exploit them.
So severance can be also understood as "we know you have the capability to harm us and we can't stop you, so to tip the scales on how much you dislike us, here's a gift of extra money beyond WARN".