It's a design that's iterated over many decades, and it hasn't always been easy for them; the payment network is huge.

Plus, if you think security is bad now, remember that before this we were writing checks for most transactions. At least with credit cards I can discover fraud usually the same day it occurs, and they are far more convenient that checks ever were -- as someone who has used both, I'm not all that bothered by the current state of the system.