For example, half of parents are transferring an average of $1,500/month, tax-free, to their adult children. Why do they get to do this?

For the same reason we have a generous gift tax exemption applicable to any gift from anyone to anyone: If you’re not receiving something of monetary value in return, what you’re providing isn’t “income” in the sense Congress has built income tax policy to capture.

That isn’t the case with tips for waitstaff.