If there was no seniority, then it would be much easier to switch between carriers. If Airline A closed your base, you could jump ship to Airline B and be based at home. If Airline A started cutting pay and benefits then you could jump to Airline C and get a pay raise.

The seniority system has been created by unions, so that they have a reason to exist. If the responsibility was on the company to provide excellent pay/QOL to attract employees then there would be little need for a union.