Variable wings increase the complexity of CG management and flight control/stability systems, as well as pilot workload (unless you add yet another system to automatically sweep wings). Plus, it's another major system that can break (asymmetric wing sweep=bad) and requires sustainment/maintenance.
This is what can happen if you have to land with wings stuck full aft:
What He ^ said.
Folding wings = cheaper, space-saving, relatively simple mechanical solution to CV life.
Sweeping wings = expensive, aerodynamic performance enhancing feature with lots of moving parts (that move while you're flying which can turn out poorly if they stop working).