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The problem with your assumption is, not everyone leaves their computers on. Most people turn off their desktops or have their laptops go into standby. CPU manufacturers have found ways to turn off parts of the CPU so it consumes less power during idle states too. Running distributed computing requires more power and higher electric bills. With today's economy, it won't take long for people to realize that the volunteer basis is costing them and many will leave.
I have a friend whom I convinced to let me put SETI on his home desktop under the premise that SETI wouldn't run while he was using the computer, and he would shut it down when he was done - essentially no computing time was going toward the project. When I asked him, he stated that he's very financially conservative and he saves every penny he can through energy efficiency.
That example is just a portion of the people out there, but I imagine it's a large portion. It requires dedication to leave your computer running and take on the extra expenses of volunteer computing.
Bottom line is that not everyone runs their computers 24/7. | 
