Why don't student visas in UK, Australia, NZ, Canada & US?
The countries you mention earn a lot of money from international students. They also benefit from bright people from overseas coming to their country and making a positive economic contribution. They are happy to offer education in whatever subjects the students want to. That does not necessarily mean there is job prospect or skill shortage in all those fields. Even in the fields where there are jobs, the countries will not want to keep the students with low academic attainment who will struggle to get a job and can become a burden to the society. So they do not want all students to stay back automatically and potentially create problem in the labour market. Therefore, they offer work visas only to those who can successfully get a job. For similar reasons, getting a job offer does not lead to permanent residency (PR) or citizenship. Just getting a job is not a guarantee that graduates will be able to stay on the job and sustain themselves. Hence a fixed duration of successful employment is required before PR and passport.