Why is the power factor always 0.8?
Power factor is not corrected to 1 because power the power factor is likely to change with changing load. With correction to 1, the power factor could become leading and that is often more difficult for the source to accommodate. It could cause the voltage to rise.

In your marine vessel example, the engine generator set may be designed for 0.8 pf. If the engine is sized to produce a certain KW and the generator is designed to produce a KVA that is 20% more than that, there is not much to be gained by adjusting to anything higher than 0.8.