Cognizant — got selected but offer not released

Address: Chennai, Tamil Nadu, 600097

Website: www.cognizant.com

I was interviwed in cognizant on 15 sep and got selected at tco chennai location the contact person was ashok.
But after that i have not received any response from them. I just got mail from congnizanttag that you have cleared your interview and will get back to you for next step.
The same thing happens in jan 2018.
Why this people are doing like this.