If every year time saving devices are invented (i.e. computers, cell phones, email, etc...) then why do we still need a 40 hour work week?

We don't if we we want to accomplish the same amount of work that we used to. But now a simple computer can do the work of a roomful of accountants and bookkeepers, a pool of secretaries or a lab full of scientists.

However, computers have the capacity to generate so much more information than the previous technologies, that management wants to use all that info, for example, to give them a competitive advantage, to boost profits, etc.

I have been through a couple of technological 'revolutions' in my working career; each one promised to save money by cutting down on labour costs, and each one has resulted in increased labour costs, because the new technology has multiplied the information that needs to be processed for management. Increased info = increased demand to do something with all that info. But don't try telling that the the big wigs making the decisions - they are still stuck in the mind-set that technology can replace employees.

They may save time, but they don't save work.

Work exists in an infinite supply in the mind of the employer.

The world isn't a fair place to begin with, so the money isn't equally devided.
Also do we have to ask ourselves whether we need everything we own?
In the fifties, we had only one lightbulb in the livingroom, there were only a few cars in the street, and we listened to the radio instead of watching TV (let alone PC's and internet). Nowadays we buy the latest of everything, designer clothes and eat meat every day.
If you wish, you can work 20 hour a week, but are you willing to give up all this luxury?