Why are people always bashing 1st round draft picks?

I happen to look through the list of first round draft picks and i must say. I was impressed that i knew most of them and they had a lot of superbowl winners on there. Surely people who have made pro bowl. I seen some flakes but for the most part i was impressed that i recognized 70% of the names (post 1970). Whats all the fuss about first over all draft picks as flops? Looks to me like its more like out of every 4, 1 will be a flop. And even less than that if you want to talk out of the league in a year or two. Hell Bo Jackson was on that list as well as John Elway and peyton manning! Hardly call those flops.