Because of the physical limitations of chemical propellant.
The bullets fired by the .30-06 are infinitely better than they were 100 years ago.
Agree! The only way to drastically improve the cartridge�s performance (with current powders) is to increase pressure. Many older guns would not happily digest dramatically higher-pressure loads; they suffer heart-burn and so would the shooter.
In fact, current guns would not happily digest them either. 
