For the same reason witness delegations expire: to ensure active users are the ones engaging with the platform. I could be persuaded delegations are different. I'm just suggesting it as a point to discuss if we're out to prevent moochers and manipulation.
Ye, I got that part.
But some people might have left Hive for good and decided to delegate their stake to a project or something.
And that would be a sound argument. However, it could be addressed with a secondary long-term form of delegation, with a revocation on the timeframe of HP powerdown to prevent abuse. I know, extra dev work for something that may not even need to exist.