When using Adobe Lightroom or DxO PhotoLab as host application I can only work with TIFFs they generate: why is it not possible to open RAWs or at least DNGs with Nik Collection?

This information applies to all versions

Nik Collection was developed as a plug-in for use with compatible host applications, and the plug-in format does not support RAW images.

Therefore it is compatible with JPEG or TIFF images using RGB color space.