That may not be necessary for all types of job. Years of training through to graduation level should be enough to prepare student for a future job and not 2-hour examination.
Host countries Like Canada and Australia accepting immigrants and international students rely on specific IELTS band scores because such tests have a great predictive validity in foreseeing the candidate's ability and performance quality in the targeted environment. Therefore, the requirement is justified in that it acts like a kind of aptitude test.
as an aptitude test with predictive validity , it can work efficiently. but the question is that whether ther eis a correlation between different majors or future jobs and IELTS constructs?
A good score on IELTS (academic, rather than general training version) gives reassurance to employers seeking EAP teachers. However, I don't think that an IELTS certificate (with a particular minimum result) should be a requirement for employment. Employers should be able to make a judgement about a person's level of English through his/her c.v., an interview, and any other tasks that are set as part of the selection process.
IELTS test does not necessarily have to be implemented at the institution itself to play its diagnostic role. It seems to have been doing its job so well though it seems to be of a speed test rather than only a general proficiency test.