They can be, and they are in certain linear PSUs. Linear PSUs generally make ripply DC with linear power transformer -> rectifier and then use active linear ana

They can be, and they are in certain linear PSUs. Linear PSUs generally make ripply DC with linear power transformer -> rectifier and then use active linear analog circuits to get clean DC power. Has very low efficiency but exceedingly low noise. For most consumer things, you use a classic switch mode power supply design. You bridge rectify mains directly, and you switch that really fast to get the DC you need, and you filter it a lot.