Because 21 million has factored the lost ones right into itself as it stands. Twenty-one million bitcoins = 2 quadrillion pennies, I mean satoshis, if the value ever gets to the point that bitcoins are traded among many people.

Bitcoin is beta. It is still in the experimental stage. Will it ever get out of the experimental stage? Nobody knows for sure. But it probably won't until people can easily trade all kinds of products for it locally.

If the time ever came that Bitcoin was in short supply because of some bitcoins being lost or hidden in some "permanent" way, The program could be altered to allow mining to start up again, if all the bitcoins had already been mined. But there would have to be need. Satoshis would have to be too "big" value-wise to be useful. We are so far away from that happening, that the only reason to talk about it is for the fun of theorizing.