It's like asking why there's no good road PBR textures, water shaders have so many different requirements and needs and layers that no one size fits all solution is going to exist. Same with roads, terrain, grass, etc.
i must have a dozen different pbr water shaders for unreal. Rivers, oceans, ponds... every type of water. All from different authors. as mentioned, of course you can transport the textures anywhere you want but the shader itself is always dependent on the language / engine.
Water is really simple as a basic PBR material. Glossy with IOR 1.333, dielectric fresnel, and refraction. The trouble is with refraction rendering, all kinds of different approaches possible, with different performance implications. Also, when any wind or physics gets involved, then you need simulation and volume…