It's a valid question. Another question could be why give pilots raises every year up to 12? It just cost them more money. Based on my observations the Years of experience are worth it. A one year Captain can be a little rough around the edges. Most people (not all) get better over time. In my time at F9 I have flown with many new Captains that have never been Captains before. What I can say is they become more efficient over time. As in they develop a rhythm to block out on time, the fly more efficiently they handle things in a more streamlined way, most importantly they mitigate risk and share experience better than most fresh Captains.

In short They are worth the extra Money. The additional experience yields efficiencies that don't show up on paper. If there is some calamity a most people would much rather have more experienced pilots up front. If you work at F9 you can look over some of the memos we have had over the past two years. UA as well. These are not mistakes I couldn't haven't or wouldn't make, but if there was more time with the two people upfront. Some of the nuanced issues may not have come up. (JFK, LAS,SJU). All three of these have been expensive problems. The over under for just money? I couldn't tell you. The public perception of a particular airline has a big effect. Reference the WN flight where the Fan/Compressor? blade came loose and put a hole in the fuselage. They lost a passenger and still came out looking good.

Back in the day there was an up and out policy at a lot of regionals it helped pad there thin margins. These days in aircraft over 100,000lbs. I really don't think your saving any money. Just the on time performance and dealing with Pax/MX issues alone a 12 year is going to pull the weight better than a 1-3 year almost always.