Because reparations are not being asked of White people. It's being asked of the government. Reparations on a personal level are only done in the case where one party or group has directly wronged another. You are correct that no one alive today was a slave or a slaveholder. However, there has been plenty of practices and atrocities since the abolishment of slavery that could be reasons for reparations for Black people still alive today or their direct descendants. And in no case would any White person personally be asked to contribute. They wouldn't even be asked to feel guilty about anything they have not done. In fact, in this regard, it's really not about White people at all. They just need to get over themselves.