it is infeasible that you will brute force a 128 bit symmetric key (simply requires material and energy on a scale the human race is utterly incapable of).
Cool!
I think people just want to know exactly how infeasible it really is... It can help confidence in bitcoin.
My thinking goes like this: if there's 2^128 combinations, then if you could try a trillion, trillion combinations
a second, it would take you 8.9 million years. I'm just wondering if we can really try that many per second.
It sounds like maybe a lot longer to try a private key than to do 1 floating point operation