Power supply physical design
Safety is a concern--moisture ingress can conduct dangerous AC voltages to the outside of the case.  There is probably also a slightly lower incidence of tampering when people know they can't easily reassemble the device.
The big driver, though, is cost.  Adding removable screws involves multiple operations per screw, whereas sonic welding is easily automated and doesn't increase the BOM in any way.  Assembly is thus easier, faster, and requires less parts.  This is the motivation for sonic welding in most cases.