I know y do it , but why !

That's really not the case overall. There's plenty of stranded wire used in AC circuits, just as there's plenty of solid wire used for DC circuits. In the RV world, the difference is primarily the ready availability of off the shelf products for the AC side, which usually mean solid wires, while the DC side is more commonly built with stranded wires to better withstand the rigors of road travel. For AC, one common exception is the stranded coiled wiring used to supply power to outlets, etc., in slides. Running all stranded AC wiring in an RV would also require running conduit per the NEC, something that's not required for low voltage DC wiring.