... contracts and building experience hoping to secure a job on the books with a decent operator, and not having left a decent on the books job to commence working as a contract pilot
I don't know anyone like this. If you're able to get a contract job it means you already have the experience as a captain in type or you wouldn't even qualify. Such people already have decent flight time and aren't interested in building hours as they are building their nest egg.
There are plenty of people who walked off "on the books" jobs to take contract work. Most airlines in the US are not such great places to work and are designed to be low paid time building jobs. You did qualify such jobs with the word "decent" so I'll give you that...but permanent full time does not automatically equate to "decent."
Most people want to work for a decent operator and there are many good jobs without seniority lists. Many such jobs start out with contract employment and end up being permanent.
There are also many very high quality international airlines that offer full time permanent jobs without the hindrance of a seniority list but I don't know of any in the US unless you get on right at launch and that's often less certain than a three year contract.
For example, Hainan airlines requires 5,000 hours total time with 1,500 in large jets (B737 size) and 500 hours PC in type. (I wouldn't recommend Hainan BTW but B737 types are in less demand than A320)
The more established the airline the longer it takes to upgrade...this is fine for low time pilots who can't get contract work but not the best choice for everyone. In the USA you don't need a lot of flight time to get an airline job as long as you meet the new minimums but some jobs are more coveted than others and the competition is higher.
Many of the major airlines in the US have a history of hiring very low time pilots from internships while bypassing experienced pilots but I don't think they're doing it so much right now. The smaller regional airlines will hire just anyone with a wet certificate and a pulse so yes...it's easier to build time in the US.
I know some people who are working as captains on overseas contracts saving enough money to be able to afford the starting salaries at the major airlines in the US until they can build up a few years to get the more reasonable salaries. Those people are high time and can already compete for the best airline jobs.
Others who are just planning to fly for five or ten years, build money fast and then retire...they're older, high time, have already flown for an airline at home and they have no intention of going back.
I even know some younger guys with outstanding experience who plan to make contract flying their whole career....it's hard to pass up a quarter million a year salary and four months vacation despite the challenges.
Almost everyone would be a highly competitive candidate anywhere so your perception of contract pilots is way off the mark.