Because if you have the money, you can insure yourself. The only reason Apple offers you insurance is because in expectation, you lose money on it. That's how insurance works, you pay a premium to avoid risk.
But with most people buying apple products, there is no real risk. You can afford to replace you computer if something bad happens to it. And over time you will save a lot of money by avoiding insurance premiums.
On the other hand, health insurance is meant to avoid catastrophic risk. Most people cannot afford to pay $100K, out of pocket to pay for open heart surgery. You still lose money in expectation, but it's worth it to avoid catastrophic risk.
Not everyone who buys a mac also has the money to buy a new one if it breaks (or pay for repair = price of new mac). I had the money to spend on the iMac 27", but I won't have that money again for another 3 years at least, so I need to make sure that when anything happens with the machine Apple will take care of it.
I do see your point though, don't get me wrong. I paid for Apple Care for both my iPhone 4 and iPad 2 (which I sold), and never needed it...
All the Apple Care I bought would buy me at least a new ipad mini... And Apple obviously is making money with that system.
On the other hand, I had to replace a 15" MBP Antiglare screen once under Apple Care which would have been about 500... so go figure.
I'd still say, if you have the money to buy Apple Care, do it. Especially for your macs which extends coverage to another 2 years (Not like the iOs devices which extend only another year)