Why can't absolute enthalpy be calculated if its components are?

[sid_galt]

My chem book says that

Enthalpy = internal energy+pressure*volume

However it says that enthalpy for a particular state cannot be calculated. Only the enthalpy change can be calculated. This has me confused.
Aren't internal energy, pressure and volume calculabe for a particular state? Then why can't we calculate absolute enthalpy?

 

[The Bob]

Enthaply is used to calculate how much energy is taken in or released by a reaction where bonds have been broken (well all reactions really need bonds to brake but still...).

Enthaply, however, also depends on the temperature (if I am correct) and ,thus, it is only possible to have an equation for the change.

This might help or checking the rest of these might as well.

The Bob (2004 ©)

 

[GCT]

It should be change in PV, or $$\Delta PV$$, the work in expansion against the atmosphere