When the government doesn't do it, it's because wealthy individuals have already bought the government. And have invested huge amounts of money into bamboozling the population into voting against their own interests.

> private parties have to

No they don't. It can also be that neither the government nor private parties give.

Making it an either/or often makes space for the individual to make excuses for why they don't share because out there somewhere there exists some government program that vaguely looks like charity.

If the government do it, because they are lobbied not to, so that private parties (with lobbyists) can, there is potential moral hazard.