HR, on the other hand, exists to create a "system." One of the primary properties of the system they are trying to create is predictability. If you have a process of some sort, anybody ought to be able to put the same inputs in and get the same outputs out.
Thus, they look for a good way to explain what you are offered. Capping it based on your previous compensation is a nice, simple rule that fits well with a process. Paying you some random amount based on how urgent your skills are needed on a team is not a repeatable process.
In a weird way, although HR talk a good line about people and soft skills and values and missions, they behave just like autistic programmers: They put numbers and rules ahead of people and priorities.
Any simple intervention that can be accomplished by HR that comes with a simple story about reducing salary costs is likely to become company policy.