It's true all bitcoins were generated at some point, but this is just the way they are initially introduced into circulation, like when dollar bills are printed.  After that they are bought and sold many many times.  So the exchanges, like many of us, have bitcoins because they bought them from people who bought them from people who.....and so on.  It's like the paper money in your pocket--do you know exactly where it came from?  You know it was printed by the government at some point, but since then it's most likely been passed around quite a bit.

I am very confused by what you mean with the phrase "the basic premise of bitcoins being generated by computer power".  The generating being done on computers is essentially the generation of security for the bitcoin network, which works precisely as designed.  In order to reward miners for their contribution to creating this security, the set number of bitcoins are being distributed in a way that rewards them.  Since mining is a task many different people can freely invest in around the world, this is much fairer than the method of most governments or banks where they simply own all the initial coins and get to spend them however they wish.  Everything in bitcoin is working precisely the way it's designed to, because competition keeps the level of security appropriate for the value of the coins.  This would be like if one dollar's worth of security measures went into each one dollar bill--it would be way more secure and difficult to counterfeit than it is currently.

Bitcoin security is created by the free and open competition of miners around the world.  For this work, they are rewarded with small portions of the pre-defined quantity of bitcoins, according to a decreasing schedule enforced from the very beginning.  This is the premise of bitcoin, and it works beautifully!