The unemployment rate would plummet if they did.
3 factors artificially keeping union workers from working even if they are offered a job;
1) Can not work for a non union company in their trade if the union runs out of work (even if they knew they didn't have any work when they "hired" the worker). This tactic is used liberally to woo journeymen from their steady non union jobs.
2) Don't have to look for work other than being available if the union calls.
3) Some unions have non compete clauses such as the one I would have had to sign had I accepted a job as a boat carpenter - I would be barred from practicing my craft in my state forever if I left that firm.