Then you underestimate a hacker.
These are the same guys that hacked into some of the top corporations, Amazon included. A good hacker can find a backdoor into any system no matter how well designed it is.
To quote Joe Sisko: "There isn't a test created a smart man can't figure out."
It's not just the fact that they were compromised--it's that, by being compromised, the attacker(s) had access to so much disparate data and infrastructure. Basic network design: you separate the various areas of your network, keeping more sensitive data in more secure locations. Anything potentially facing the outside world should be locked up tight.
I'm not saying it's possible for Sony to have ever been 100% immune to this kind of breach, but given the timeline and the amount of data that was taken, it's pretty clear that once the attacker(s) got in, they had access to damn near everything. This means whatever trust system was in place was fundamentally broken.
The fact that Sony is having to completely rebuild the PSN speaks to the fact that their infrastructure was flawed beyond repair. It's unfortunate that they had to suffer an attack of this magnitude in order to get their shit together, and even more unfortunate for their customers, who no doubt trusted Sony to be responsible in the first place.