Because there is GRayTracingSceneCaptures variable in the source code of the last UE4 engine version, which you can use it to enable the capture. It is just supported in recent.
static int32 GRayTracingSceneCaptures = 1;
static FAutoConsoleVariableRef CVarRayTracingSceneCaptures(
TEXT("r.RayTracing.SceneCaptures"),
GRayTracingSceneCaptures,
TEXT("Enable ray tracing in scene captures.
")
TEXT(" 0: off
")
TEXT(" 1: on (default)"),
ECVF_RenderThreadSafe);