Why hasn't anyone cracked a Bitcoin private key?

I know I'm not the only person to wonder this. If the American NSA, and other spy agencies can crack other encrypted things, why cant they crack a single Bitcoin private key? I suspect the answer will have something to do with "collisions" even though even though I really don't know what that means...

Switchblade:

https://steemit.com/how2/@switchblade/should-i-make-another-video

https://steemit.com/inspirational/@switchblade/cvo-motion

https://steemit.com/giveaway/@mdub/giveaway-1usd-worth-of-bitcoin-ethereum#@switchblade/re-mdub-giveaway-1usd-worth-of-bitcoin-ethereum-20160930t083958430z

https://steemit.com/apps/@peppernrino/portableapps-com-v14-2-released#@switchblade/re-peppernrino-portableapps-com-v14-2-released-20160929t060759064z

https://steemit.com/technical-analysis/@ausbitbank/steem-hits-usd3-20-steem-dollars-at-usd1-60-read-before-you-buy-a-boat#@switchblade/re-ausbitbank-steem-hits-usd3-20-steem-dollars-at-usd1-60-read-before-you-buy-a-boat-20160929t055906016z