AD & Production environments at banks

One thing I have heard a lot of people talking about in the banks are the issues around shared production environments (yes, yes, dev environments as well, but I am talking about the stuff that loses businesses money in real-time). Recent gems include:

• real-time calculation engines, batch and process based applications, and web applications running in the same memory space on a single VM.
• distinct applications running on their own JVM, but communicating across a common messaging hub whereby a huge amount of Java objects are serialized/deserialized, taking up to 90% of the CPU cycles, with 10% being left for actual business functionality

How does this happen ? Is it bad architects ? Are there historical / evolutionary reasons ? Is it bad implementations of the design patterns (if there are any) ? A lot of "bubble diagrams" and no prototyping ? A bit of everything, I suspect. I understand when tactical systems become less-tactical and more entranched, bad things do happen. But on new builds ? Seriously, when these guys are putting together new component-based architectures (in one client now), how does this happen consistently? There is a fundamental problem with the way these organizations build even the most basic of production applications. Call it incompetance, lack of quality control, poor engineering talent, bad management, or whatever, it IS incredible...