For the same reason they didn't use SHA1024 or SHA2048 or SHA4048 or SHA1000000000000000000000000000000000

There are lots of theoretical attacks that can be done against it, but if a program or new math proof can half the amount of time it takes to crack it, are we really worried about the encryption taking 100 billion years to crack but now with this new attack (insert math,attack,flaw) it's only going to take only 1 billion years to crack? How about a million years? Even one-hundred thousand years?

When they can crack SHA256 in under 10 minutes, I'll be worried, but until then, time is on your side.