Why don't we have 256-bit security level digital signature schemes (in common use)? Recently (2016) I've been researching on post-quantum digital signature schemes, and read about BLISS, BLZZRD, REBLISS schemes. What I've noticed about them is that they lack a parameter set for 256-bit security level. 

I went to the Wikipedia page on NSA Suite B and learned that CNSSP-15 require only 384-bit curve for digital signature. And then I clicked on this link in the reference section at the bottom of the page, and found out only ECDSA-with-SHA{256,386} required for the profile. 

It baffles me. Why is 192-bit security sufficient (upper-limit if you will) for digital signatures when we have 256-bit variant of AES? And what reference backs this up?
The trivial answer is that there doesn't exist any relevant security threats that entail a requirement for long term authentication. There do however exist relevant security threats that warrant long term confidentiality.

Assume you have a document and need to prove its authenticity. If you apply a digital signature to the document, but the digital signature scheme you use is later broken, it might still be possible for you to later reestablish the authenticity of the document using other means. For instance, there might be external witnesses that can vouch for the authenticity, the document might be recorded in multiple locations, making forgeries reasonably unlikely, etc.

However, if you encrypt the document for confidentiality and the encryption scheme is broken, there is no way for you to ever reestablish reasonable confidentiality guarantees. If the document is no longer confidential, re-encrypting it obviously won't restore confidentiality.