Enron had a business model that simply didn't add up. It constructed an essentially fictional view of the company and its accounts, apparently with the active support of consultants and the connivance of auditors, thus allowing the executives to maintain a fraudulent posture of success. (Obviously it helped that the consultants and the external auditors worked for the same firm, but this is only part of the story.)
In contrast, Parmalat appears to have had a viable business model. Its problem was that someone was creaming cash out of the operation - simple theft, possibly compounded by accounting laziness. (Why didn't anyone bother to check the bank account where the cash was supposedly deposited?)
WorldCom was probably somewhere between the two. An unstable business model, which allowed for plenty of bezzle in good time, but was not robust enough to withstand bezzle when the markets turned.
There is an important distinction here between direct theft (where some dishonest individuals steal from the company) and indirect theft (where shareholders are ripped off by a criminal gang of senior managers through corrupt governance).