Why don't we have 256-bit security level digital signature schemes (in common use)?
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.