Because their secret technology is 50-100 years ahead of any public perception of technology. They are no fools, they have the algorithms to decrypt any public cryptography cipher on which bitcoin is based. They also now have very powerful supercomputers so even if you manage to develop your own cipher they will crack it in seconds.
Cracking a cipher isn't doable since the digest is smaller than the input, significantly. The only way to do it is by trying every possible keypair. There are 2^256 possible keypairs.
No the concern is still valid.
Not really. Imagine you built a perfect computer; forget about GHash and Megaherts. You built a computer which used the absolute minimum amount of energy theoretically possible to record a change in a single bit (1 to 0 or 0 to 1). We are talking about the limits of thermodynamics; nothing more efficient is even possible. Now imagine you used most of the natural resources in our star system to construct a dyson sphere and covered the entire surface of this sphere with a single star system sized super computer. Now imagine you could keep this supercomputer cooled at roughly absolute zero and could do so without expending any additional energy.
If you had that and captured (with no inefficiency or loss) the entire energy output of our star (not just in a day or week but continually until it burned out) you couldn't
COUNT to 2^256 before you ran out of energy. Keep in mind this is simply counting. Just counting, not hashing, not comparing, not performing lookups just counting 1 .. 2 .. 3 .. .... 2^256-1.
This program couldn't finish even using all the energy in our star system
Int256 i = 0;
while (i < Int256.Max)
{
i++
}
Print("Congrats we counted to 2^256")
Or put another way:
These numbers have nothing to do with the technology of the devices; they are the maximums that thermodynamics will allow. And they strongly imply that brute-force attacks against 256-bit keys will be infeasible until computers are built from something other than matter and occupy something other than space.