Two great answers above cover most issues.
Need to add in preponderance of overseas registered airlines that actively discourage/ban union membership - Ryanair.
Low Cost airlines love cadets because they cost sod all. They pay for their own training and will then work for peanuts on a zero hours contract. Displaced experienced pilots (XL/Viking/FlyBe/ThomasCook/Monarch/SilverJet/Many more) struggle to get a seat in remaining airlines unless they are prepared to have terms raped to unaffordable levels. They then have to drag their families to the middle east or worse where the money is fine but that is about all.