You might be looking for discussion and not argument, but your premise of how/why collective bargaining agreements are negotiated is one borne in both ignorance and assumption.
Retirement plans: years ago, pensions were commonplace across industries. Only in the last couple decades have defined benefit pensions largely gone away in favor of 401ks with a meager company match, and today VERY few companies still have defined benefit plans and almost all of those are due to collective bargaining agreements. DB plans are expensive to fund, especially following 9/11 bankruptcies and dumping of plans onto the PBGC. Unionized airlines have negotiated for a defined compensation retirement where X percent of their income gets deposited into an account by the company into an account owned by the employee as a hedge against future bankruptcy losses, but few employers outside these unionized companies have such DC contributions (again, most companies only offer a 3-6% 401k match).
Job portability: seniority means everything, especially in aviation. Let's say I go to Airline X and you go to Airline Y, and I start class one day before you do. Airline Y grows like gangbusters and Airline X is stagnant; five years after hire you are a Captain at Airline Y and Airline X folds. Would you be okay if I just slid over in front of you, taking your schedules and vacation? I'm gonna guess probably not, as you made a 'good' career choice and I did not. Protecting what you have is base human instinct, and in the early days of commercial aviation airlines were starting and collapsing all the time - what you see today has come from that.
There's also the issue of FAA operating certificates; while electricians are mostly 'plug and play' from one job to another, a pilot that flies a 737 for Airline X can't just sit in a 737 at Airline Y and go fly. That pilot would need to go through and successfully complete a training program from Day 1 Basic Indoctrination through IOE and Release to the Line Check.
Besides this, where is the incentive for an airline (we're educated in the ways of economics and business philosophy, right?) to hire somebody who is at year 12 on a pay scale who simply is looking for work, when they could hire somebody at year 1 that actually wants to work somewhere?
There is no national seniority list for the airline world, and the manner in which one would go about creating one would be FAR more divisive than current "siloed" Locals are.
Also, trade unions like Teamsters, UAW, IBEW, etc. fall under NLRB, not RLA like airline unions do, and the bargaining processes of the two are similar in some ways but starkly different in others.