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.

Power factors are often "corrected." Typically, line loads from electric companies are inductive loads (transformers, high power industrial motors, etc). So the solution is to have some giant capacitor, which is not terribly uncommon.

Why don't we correct everything? It's not ever possible, and usually it's not practical. Inductance is not constant. E.g. night time, there is less power going to industrial machines so the inductive load may go down. If you correct the P.F. for these inductive loads, your P.F. will change again when those loads are turned off.

In fact, power factor is corrected in many cases to reduce line currents and the associated distribution losses. Large industrial main loads are required or incentivized by the utility company to reach a minimum power factor by installing capacitors or active PFC.

Active PFC is starting to show up in consumer electrics, as well, to improve the situation from the buttom. Some "green" certifications, such as certain Energy Star certifications, specificy a minimum PF. You can find off the shelf LED bulbs that feature active PFC in order to reduce the harmonic currents that are generally introduced by rectifiers.

For more information, see the following links:

https://en.wikipedia.org/wiki/Power_factor#Importance_of_power_factor_in_distribution_systems

http://www.eaton.com/ecm/groups/public/@pub/@electrical/documents/content/sa02607001e.pdf

http://powerelectronics.com/power-management/power-factor-correction-justified-home