The rationale for these kind of severance packages is that this is what it takes to hire a top executive away from a top company and into a entirely new role at a riskier company. When an executive leaves a top job to join a new, undefined, risky role at a riskier company, they do so knowing: 1) they will not get that (relatively safer) top job back, a job that represents all the years they spent ascending the org chart, 2) they might not survive in their new job, especially if it comes with performance targets [1], and 3) getting fired for poor performance will permanently tarnish their (previously skyrocketing?) reputation. With that in mind, these executives will always get a golden parachute in their contract.

[1] I'd be interested in seeing data on how many executives do survive in these situations. I wouldn't be surprised if the data showed that a large (~40%) proportion do not hold onto their positions.