In Australia it's not negotiable. Our employment laws dictate what severance payment is required if a company is letting people go to reduce costs. Those laws don't apply if the employee leaves of their own accord, or if they are fired for other reasons.
Then the situation is maybe not so different? It's an amount you're entitled to when laid off, set by the government, which your employer has to fund proactively (payroll taxes).
Because it's set by the government, it caps out at well below a typical tech worker's burn rate, so it's little more than a brake on the depletion of our savings. When a tech company offers severance it's typically much, much more than what the government guarantees.
You also get the full amount regardless of how long you're unemployed, so e.g. if you get laid off with 7 weeks severance and find another job in 3 weeks, that's a free month of salary.
In our case, the payout is defined as 4-12 weeks of salary (plus 1-4 weeks notice), based on length of tenure. Since this scales with what people earn, there's little need for tech companies (or other well-paid fields) to offer anything extra.
On a related note, Australians are also entitled to 4 weeks paid leave per yer, which accrues if not taken. Employers are also required to pay out any leave balance (even if the employee leave voluntarily), which means most workers will have at least a couple of additional weeks paid out on top of the severance.
It's the same here, in that you get the full amount regardless of how long you're unemployed after they let you go.
