Why are people not voluntarily donating to it part of their rewards after posts pay out?

You're holding them ransom with the unwritten rule that they'll get the votes if they set your account as beneficiary, technically self-voting for the "growth" of the account. A quick look at the voting reflects that they mainly look for posts with the beneficiary to vote on.

Not even gonna get into the selfishness of growing the account and the amount of rewards it'll need in the future to maintain that growth along with you hoping many delegators will just be too lazy to remove the delegations if the returns that are unfair to begin with. It'd be one thing to promise delegators more rewards if that came from your own pocket, that would show actual care for the community's growth but it's another thing having the community pay for them.

It's all just a wicked scheme and somehow you've tricked a whole community to fall for it, thankfully the rest of Hive aren't as easily tricked and can see your intentions and schemes and won't tolerate it.