Why some jobs determine nationality of the applicant? isn't it racist?

Depends, if you have anti-discrimination laws. 

Dear Ibrahim,

I agree with you such type of advertisement you mentioned is exact racism, when they mention particular nationality. Regarding anti-discrimination laws ,i believe none of gulf countries or Arab countries they don't have, if any country have such law it will be our pleasure .

Dear Mohamed if the there is such a law like in kuwait or saudia  mostly they have reasons but im not talking about this type im talking about something as following 

One of the leading fashion retail company in Kuwait is actively looking for the following : - Human Resource Manager . - Experience (5 -8) years . - Arab Nationals only (prefer Syrian - Lebanese - Jordanian).

maybe in this advertise a little bit  open chance for other nationalities  but there is something more like pointing to Specific nationality i think this is a kind of racist.

Depends, if you have anti-discrimination laws. UAE has no such law as yet. 

also, There are some jobs which may require a specific regional person for better output/results so its not that every organization is discriminating. 

Depends, if you have anti-discrimination laws. UAE has no such law as yet. 

also, There are some jobs which may require a specific regional person for better output/results so its not that every organization is discriminating. 

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The reason behind that could be because:

1. The company requires certain nationalities for work.

2. For legal and documents facilitation