Why are airline pilots not laterally mobile?

In most industries when you want to switch companies you can do so without much difficulty and often with a raise in salary. With airline pilots, I’ve read that they are stuck because the industry does everything by seniority. For example: If you’re a senior captain at Airline A and decide to move to Airline B, you have to start over from the bottom of the ladder as a First Officer at Airline B simply because you are new to the company and have no seniority there. Why doesn’t seniority transfer and why don’t unions fight for this for their pilots?