Why do government conduct exams in omr method instead of cbt??

I had written exams both through cbt and omr modes. I find cbt mode convenient it saves our time. And many a times when I review my answers after answering all the questions I have changed my wrong answers to correct ones in cbt mode. But in omr mode when I review I find more answers wrong with no chance to change them. The worst part is I found that I have marked 10 wrong answers but was unable to change it. We have so much technological advancements but still we have this out dated omr mode in many exams. Is it only me who finds it difficult with omr??