In my (thankfully limited) experience, it's entirely standard for severance packages to be tenure-based and, IMO, most of those reasons have nothing to do with ageism.

The longer you've been with us, the more you've likely contributed and we should recognize that as we part ways. The longer you've worked with us, the more life upheaval the separation represents. The longer you've worked with us, the more personal relationships you have developed, perhaps with the same people making the decision to eliminate your position. A generous severance (which IMO requires a tenure component) sends an important signal of stability to the employees who remain AND makes it easier for managers (who are of course humans themself) to make the decision to eliminate the position of a long-tenured but "surplus to ongoing operations" employee.

What's the alternative? The person who joined 6 weeks ago gets the same package as the 10-year veteran? That seems worse all around to me.

There is a subtle difference between company wide push to get rid of employees and severing employment on an individual basis. Giving presumable older people more in the first situation looks like a systematic effort to get rid of them.