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Does anyone know why the water and oil must come from the engine when running lines to the turbo? Is there any reason why the turbo couldn't have it's own water and oil supply?

I guess there is no reason it couldn't have its own supply. The main reasons I can see why the engines supply is used would be space, economics, reliability and Pressure.

Space: Where are you going to mount external fluid reservoirs?
Economics: You are already using two pump and filtration systems, why make another one to do the same job?
Reliability: The more pumps you use the higher the chance of failure.
Pressure: As the engine revs, the turbo spins faster and so does the oil pump and water pump to supply the required cooling, you would need countless sensors and more wiring and some sort of ECM to do the exact same job every basic engine was designed to do.

theres no reason you couldnt run separate systems, but i dont see the point, as theres already ideal sources of both already within inches of the turbo.

In the early days this was done because of some unusaul places they stuck turbo's.But it is extra work which is not necessary.