Why its so difficult to get a job offer or even interview calls through online portals here in UAE ?

Due to high number of applicants being applied through online portal and it's not practically possible for recuriters to go through individual CV's and look for potential required to fill the vacancy. 

If I have understood your question correctly, from experience there are such vast numbers of applicants for any job advertised that it would be close to impossible for the recruiting manager/HR to contact all applicants to provide feedback. Coupled with the fact that there seems to be a lack of online job application management systems which allow several applicants to have the same action applied to them at once (for example rejections, invite to interview etc) you are facing a losing battle if you do expect to receive feedback on every application you make.