Why does the neutral wire don't connect straight to earth but need to earthed only in substation?

Answer 1

I've never heard of a power distribution system which does that. The neutral line is usually grounded only at a building service entrance. The output of the final stepdown transformer is DC-isolated from its input. Areas with high lightning activity often have a grounded wire above the wires carrying the power. They are usually grounded at every tower.

Within a building, grounding is usually done only at the service entrance to avoid ground loops, which can cause problems with electromagnetic interference.

Answer 2

for safety reasons and ground wire leads to the ground
neutral completes the circuit.