Simple. The code came that way from bytecoin, from which MRO was forked (or alternately relaunched), and was supposedly going for two years. So somehow that little gem was "missed" by them for two years.  When the Monero team discovered the issue, we released it, and then most of the other cryptonote coins copied it, as I said.

Were it not for the Monero team fixing it, would it have been allowed to continue for another two years?

EDIT: And no, we did not carefully review 100K lines of code before the fork/relaunch. If we did you would still be waiting for the coin.  That work is still under way. For that matter the other CryptoNote coins are doing the same thing. We all push out fixes and improvements as we find them, often adopted by all.