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.
I would like to see that statement qualified with model numbers, math, and source data.
Like that's going to happen
The 30-06 is registered with SAAMI at 60,000 psi max average and proof between 78,000 and 84,000 psi.
The 30-06 case head, the 1889 7.65x53mm Mauser case head when built with a large primer pocket is good for 65,000 psi in production, and reloaded at least once without primers to falling out.
I am not aware of any 30-06 rifles that are safe at 60,000 psi, but not at 65,000 psi.
There is much safety margin in the rifles, and little in the brass.
The brass is the weak link.
Focus on the brass.
30-06 rifles being old is a red herring.
After a lifetime of living in fear and ignorance spawned by mediocre load books, these are concepts difficult for some old men to internalize.... they don't get it.