Airlines want the PIC time because they want to hire future captains.

If you have 1000 turbine PIC, it would be a reasonable assumption to make that you have had to make command decisions, exercised judgment, and demonstrated leadership.

If you have only SIC time, you may be a great pilot, but your experience as the leader and decision maker are more limited.

I know this is not perfect, but that is the logic behind the PIC requirement.