- You are getting paid to interview

- If they decline, you usually get a very generous severence package

- While sometimes technical, there is usually significantly evidence of your abilities from the acquired company's records.

- In my experience it's usually a cultural test more than any other.

- Interviewing is part of the evaluation. If the hiring company thinks everyone is grossly incompetent they can reevaluate the acquisition or call it off entirely.

Not all of it, but some of the force does to ensure that they meet the bar of the hiring company and also to get calibrated in the eng ladder.