Well there’s cost. Think about how many outlets in an office or home.
You may not understand the engineering of “UL” or safe switches because they would have to have full current of a short going through them, you have arcing, lifetime toggles, the ozone weakening the molded plastic, etc.
With most outlets (receptacles) there is a solid copper bar that goes from the wall wires to the part that pushes against the prongs of the plug. With a switch it’s slightly more complicated and would you only break the hot? What if someone wired outlet backwards?
Sure you could make every plug involve a fuse, circuit breaker, arc fault, GFCI, and a switch. And a USB transformer and nightlight.
It could help though in many homes if you had only one outlet wired to a switch, and could help with parasitic current draws of almost everything modern.
You wouldn’t because they don’t exist outside of switch in place of top outlet you could wire to control one outlet.
But honestly the US already has switches on many outlets, because GFCI is required near water and some bedrooms, so the GFCI outlet can act like a switch with the test button