Why mine tubes are not ready-made rocket launchers

Could submarine mine tubes have launched solid-fuel rockets against land targets?

Only after a substantial redesign. Rocket launch stresses and requirements differ too much for mine tubes to be treated as the same system.

Enlarged First World War mine tubes could not simply become solid-fuel rocket launchers. A rocket would have to be designed for the tube, and the submarine structure would need to handle a different launch event. Mine release and rocket firing impose different stresses, material demands, and safety problems on the hull. A workable weapon might occupy a similar position and still use a tube, but it would be a new system rather than a converted mine installation.