Cognizant — offer letter not received (4 months)

I have attended interview with cognizant on dec 2018. It's been 4 months. But still i'm not received my offer letter. Meanwhile, i'm getting calls from hr asking about still interested to join cognizant. I sent mails to hr so many times, and yet no response from them. Is it because of notice period (90 days).? If you are not ok with the notice period why can't tell that during the interview? What is the use of selecting candidates and making them wait for the offer letter?