Bounty is not a job because it is not a job? It is job if it brings profit, for many it is main source of income so for many it is a job.
Sorry for the interruption,bounties are giving rewards for the completing task which doesn't mean profits.
When we need to call something as a job it needs to have stable income,if not then it can be a profession.
But bounties may or may not exist in the future which is already not worth so don't consider it as job.