It's because we are unionized, in a nutshell. And the a union represents the interests of the majority of its' membership.
The answer lies beyond the concept of a regional often being a 'temporary' job. It's more of an industry standard that goes way back, not only in the regionals but also in the majors.
It's also related to our travel benefits and our schedules, both of which make it (relatively) easy to "commute" long distances to work. Most people in other types of jobs don't even consider the possibility of commuting by air from hundreds or thousands of miles like many do in the airline industry.
If one pilot is senior enough and chooses to live at a lower-cost domicile or to commute from a lower-cost area, he doesn't want to give up wages (be paid less) than another pilot who often chooses to live in a higher cost domicile.
Consequently, many pilots who do not have family "roots" in those higher-cost domiciles often commute to avoid the high cost of living, the congested cities, or both.
So airline unions have typically not negotiated contracts based on the local cost of living because often pilots live in those areas by choice and the majority doesn't want to subsidize the minority.
If airline pilots were not unionized, perhaps our companies might allow for cost of living in setting our wages as is sometimes practiced in other industries.