Actually they *are* lost. If you lose your private key, nobody else will be able to spend them anymore.
So, their value will go in the still existing bitcoins.
The fact is that it is hard to distinguish between a "lost" bitcoin and a simply "held" one. You can just do it statistically.
That's not entirely true.
Technically you could brute-force the private key and thus gain access to the wallet, but it is highly dependent on luck and the timeframe for actually getting the private key is large enough to be unviable and it is far more profitable to mine with that processing power than try to brute force a private key. But then again, knowing how computer technology advances it may be possible in the future at some point.