Here's my theory (based on absolutely no evidence whatsoever, it just sounds good).

The crypto market is run by the NSA. The 'difficult' sums that miners do is really factorising large prime numbers, the ones used to encrypt everything from email to online banking.

It's extremely dificult to factorise large prime numbers, unless you have a quantum computer (that's coming...) or you have a very large botnet of able machines that crunch numbers all day long...

I totally just made all that up, but you never know... woooo !