Why is it that it takes one to study hard when it comes to job offers they are restricted to nationals?

this is due to legislations, also mentalities, and sometimes  political reasons, some countries may have  agreements between them on that purpose.

and this is frustrating, true

we all get used for that

Its indeed very depressing when you go through all those lengthy job descriptions that you satisfy and at the end those jobs are only for certain nationalities. I agree and share your sorrows.