They don't have to. In federal law (and in several states), you're guilty of attempt if prosecutors can prove beyond a reasonable doubt that you had the intent to commit a crime, and took some substantial step towards it.

They've more than cleared that bar here.

Even if there is no actual target?

For example: Is the rational that if I earnestly believed I was planning on murdering someone who I thought existed, that it is the lead up and consideration that is the crime, rather than the actual attempt (which could never happen)?

Yes. That's why we have a crime of "attempt". You do not need to have a likelihood of success; it only needs to be proven that you did intend, and that you took substantial steps. Ulbricht spent more money on these crimes than most people earn in 10 years.