If work opportunities dry up, how the union would help? Unions don't create work opportunities, they only can redistribute who gets the ones that are still there - e.g. by excluding nonmembers or by mandating that less senior members will be fired first, etc. Given that, why "these people" would be sure they aren't ones who'd get the short straw?
Some of these people truly, truly believe that they are so gifted and so talented that the work opportunities will simply never dry up.