Oh I know...dont worry dude we have all become quite accustomed to your lack of understanding.
But the concept is simple....with the numbers drafted, its more likely that a lesser talent gets a shot in the NFL as a corner than at other positions.
using some arbitrary imaginary rating system... lets say a team needs a WR, an edge, and a CB.... but when their pick comes around...all the guys on their board are rated at 55 out of 100...teams will view the edge and the wr as wasted picks, certain busts...but they will pick the corner, because he is fast, or looks to be a system fit, and they can coach him up...and with the passing volume in the league combined with the cap cost of a good corner...taking a risk on a cost controlled chance for corner is more palatable...and if a guy is weak in certain situations a good DC can mask those weaknesses with safety help over the top.