I don't think a phone needs to be 100% waterproof to 50 metres, but some amount of water-resistance is just good product design. Companies should not be encouraging people to dunk their phones in water, I think IP ratings sadly encourage this as well as some of the advertising around their water resistance claims.

But having some amount of water resistance built into electronics helps reduce e-waste because accidents do happen. For more serious water activities people should be buying waterproof bags for their electronics.