Why do we need long-term schedulers? wouldn't be possible to handle the same job using
short-term schedulers? why?

The scheduling process in the system is done by using the long-term scheduler and short-time scheduler. The long term scheduler are those which select the work from the job queue and the job selection is not very frequent.

Short-term scheduler is the scheduler that select the work from the ready queue and allocates it to the CPU(Central processing unit).It works by picking the work at quick frequency .So, it is not possible for the short-term schedulers to handle the job because of the different frequency of work.