The .45 ACP is perfect just the way it is. It was designed to be as low pressure as they could manage, because it�s a military cartridge and lower pressure is always preferred over higher pressure for military applications for a lot of reasons.
In the case of the .45 ACP and most other handgun cartridges pressure is incidental and important only as a factor in bolt thrust. Parts will start breaking down due to bolt thrust long before a barrel suffers pressure damage.
The top handguns during the development of the .45 ACP ran at around 3,500 lbs. of thrust. My guess is that Browning used that as his design limit. Since the .451" diameter was already mandated the pressure was limited by default.
